The science behind Witty

Strategies to limit degradation and maximize Li-ion battery service lifetime - Critical review and guidance for stakeholders
Maxwell Woodya, Maryam Arbabzadeha,1, Geoffrey M. Lewisa, Gregory A. Keoleiana, ⁎, Anna Stefanopouloub, Cadex Electronics Inc.
a Center for Sustainable Systems, School for Environment and Sustainability, University of Michigan, 440 Church St. Ann Arbor, MI 48109, USA
b University of Michigan Energy Institute, 2301 Bonisteel Blvd. Ann Arbor, MI 48109, USA

Abstract

The relationship between battery operation and their degradation and service life is complex and not well synthesized or communicated. There is a resulting lack of awareness about best practices that influence service life and degradation. Battery degradation causes premature replacement or product retirement, resulting in environmental burdens from producing and processing new battery materials, as well as early end-of-life burdens. It also imposes a significant cost on the user, as batteries can contribute to over 25% of the product cost for consumer electronics, over 35% for electric vehicles, and over 50% for power tools. We review and present mechanisms, methods, and guidelines focused on preserving battery health and limiting degradation. The review includes academic literature as well as reports and information published by industry. The goal is to provide practical guidance, metrics, and methods to improve environmental performance of battery systems used in electronics (i.e., cellphones and laptops), vehicles, and cordless power tools.

1. Introduction

Lithium-ion batteries (LIBs) are currently the most widely applied technology for mobile energy storage, and are commonly used in cellphones, computers, power tools, and electric vehicles (EVs). Battery degradation occurs both over time (calendar aging) and with use (cycling aging), and is related to battery chemistry, environmental conditions, and use patterns. Limiting degradation has been identified as one of the green principles for responsible battery management [1], as extending battery lifetime decreases costs and environmental burdens associated with the production of new batteries, including material consumption, mining impacts, and greenhouse gas (GHG) emissions [2]. As the mobile electronics and EV industries continue to grow [3], even small improvements in lifetime extension will have significant environmental benefits. Understanding the operating principles and degradation mechanisms of LIBs helps elucidate behaviors that can extend battery lifetime. From this review of academic literature, these degradation mechanisms and relevant variables are identified. These variables are then compared with user guides, user manuals, and publicly available battery information provided by manufacturers. Finally, through the distillation of these sources, we develop and present a list of best practices for battery lifetime extension. The remainder of Section 1 describes the operation of and most common materials used in LIBs. Section 2 shows mechanisms by which LIBs degrade and Section 3 illustrates the impact different conditions or variables have on degradation. In Section 4, a comparison between manufacturer instructions and academic literature about battery degradation is made. Section 5 details how degradation is managed, by battery management systems (BMSs), and by users. Here the information in previous sections is synthesized to create a list of best practices for battery lifetime extension. This list is intended to guide users and is presented alongside information showing that currently users either do not know or do not follow many of the behaviors that can extend battery lifetime. Educating the public on these best practices is a primary motivation for this work. A battery cell consists of positive and negative electrodes and an electrolyte that reacts with each electrode. When a battery is discharging, the negative electrode (anode) is oxidized by the electrolyte, freeing electrons from the anode material. Electrons from the anode flow through an external circuit powering a device, to the positive electrode (cathode).
At the cathode, the metal oxide is reduced, gaining electrons from the external circuit. Charge is conserved at both electrodes by the flow of lithium ions from the anode to the cathode. These ions intercalate into the lattice of each electrode. The electrolyte is ionically conductive but insulating to the flow of electrons, to ensure the electrons flow through the external circuit, preventing self-discharge. A porous separator physically separates the positive and negative electrodes to prevent short circuits, while allowing the flow of ions. This process is shown in Fig. 1. To charge the battery, a voltage is applied to the circuit, and the process moves in the opposite direction. Material choice is a key variable in battery cost, performance, and function, and a variety of materials are currently used. The positive current collector is typically aluminum coated with cathode material. The negative current collector is typically copper coated with anode material. The separator is typically a polyolefin plastic, such as polypropylene (PP) or polyethylene (PE), though ceramic blends have also been used [4,5]. The cathode is typically a metal oxide. The choice of cathode material, along with anode material choices, will impact nominal voltage, cycle life, self-discharge rate, specific energy, specific power, energy density, power density, operating temperature range, and cost [6]. Commercially available cathodes are lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium iron phosphate (LFP), lithium nickel cobalt aluminum oxide (NCA), and lithium nickel manganese cobalt oxide (NMC), and composite blends of these materials [7,8]. Anodes are typically some form of carbon, usually graphite. One emerging anode material is lithium titanate (LTO). Compared to carbon anodes, LTO has low energy density but high power density. Though it is currently a more expensive option, it has a higher cycle life and can operate at lower temperatures than traditional carbon anodes [7]. Lithium metal alloys, including lithium-tin and lithium-silicon, have a much higher theoretical capacity than graphite, but large volume changes when cycling have impeded commercialization of these technologies [9].

Fig. 1: Flow of electrons and lithium ions and reactions at each electrode during battery discharge. As the battery discharges, Li in the anode (x) decreases and Li in the cathode (y) increases. C corresponds to the battery state of charge and the relationship between x and y depends on the ratio of active material between anode and cathode.

A binder such as polyvinylidene fluoride (PVDF) is used to bind the particles within each electrode to a conductive additive, ensuring the entire electrode is conductive [10]. The cathode and anode are immersed in a gel or liquid electrolyte, consisting of a lithium salt dissolved in a mixture of organic solvents. The most common lithium salt is lithium hexafluorophosphate (LiPF6), though lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), and lithium perchlorate (LiClO4) have been used [11]. Common solvent mixtures include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethylmethyl carbonate (EMC) [12]. Cathode, anode, and electrolyte materials are all active areas of research, with battery lifetime as one of many performance metrics that can be improved [13]. The most common commercially available battery materials are shown in Table 1.

2. Lithium-ion battery degradation

2.1 Characterization

There are two main forms of battery degradation: capacity fade and power fade. Capacity fade is a decrease in the amount of energy a battery can store. It is measured as a battery's capacity (amp-hours) relative to when the battery was new, expressed as a percentage. For most products, 20% capacity fade (80% of initial battery capacity) is considered the battery's end of life (EoL) [14]. The rate of capacity loss is significantly dependent on charging/discharging conditions, including maximum voltage, depth of discharge (DoD), current and load profiles, and temperature [15]. Power fade is a decrease in the amount of power a battery can provide due to an increase in the battery's internal impedance (resistance - measured in ohms). Capacity fade and power fade can occur simultaneously. To understand the precise mechanisms that lead to these forms of degradation, both in-situ (including in-operando) and ex-situ (post-mortem) characterization techniques are used [16]. These include atomic force microscopy (AFM) [17], electrochemical impedance spectroscopy (EIS) [18], focused ion beam scanning electron microscopy (FIB-SEM) [19,20], Fourier transform infrared spectroscopy (FTIR) [21], Raman spectroscopy [22], transmission electron microscopy (TEM) [23], X-ray diffraction (XRD) [24], and a wide range of combinations of these methods and emerging techniques [25].

2.2 Modes and mechanisms

The aging mechanism and cycle life depend on the battery's cathode and anode material [26]. Battery degradation is complex, as different factors from environmental conditions to product utilization patterns interact to generate different aging effects [27]. Degradation can also take place during rest periods, when energy is not being drawn from the battery [28]. The major degradation modes in LIBs are loss of lithium inventory (LLI) and loss of active material (LAM) [26]. Loss of lithium inventory is a decrease in the amount of cyclable lithium in the battery. As lithium is consumed in side reactions, it is no longer available to intercalate into the electrodes, decreasing battery capacity. Loss of active material results from degradation of electrodes, reducing the number of sites available for lithium intercalation. This leads to both capacity fade and power fade, and occurs at both the anode and cathode [29]. Capacity fade from LLI and from LAM are not additive; the overall degradation is a function of the dominant mechanism. Conversely, power fade is the summation of the impact from LLI and LAM [30]. Degradation impacts every part of a battery. In addition to the active materials, inactive components (e.g., binder, current collectors, separator) all degrade with time and use. There are many processes contributing to the degradation of each component, and it is a challenge to study these processes individually, as they occur on similar time scales and interact with one another [31]. Nevertheless, there have been many experimental studies on each of these degradation processes, focusing on both mechanisms by which they degrade the battery, and variables that influence the degradation.

2.2.1 Anode degradation

The major mechanisms for anode degradation are solid electrolyte interphase (SEI) formation, metallic lithium plating, and loss of active material. Batteries are assembled in a discharged state, since lithiated carbon is not stable in air [32]. Therefore all of the lithium ions are initially in the electrolyte or intercalated in the cathode [33]. When the battery is cycled for the first time, lithium ions from the cathode along with organic compounds from the electrolyte solvent react with the graphite anode creating a thin film called the SEI [33]. The creation of the SEI irreversibly consumes lithium, decreasing the lithium inventory available for cycling, and reducing battery capacity [34]. SEI formation happens during the first several cycles coating the graphite electrode with a film tens to hundreds of angstroms thick [32]. This film consists of organic salts, inorganic salts, and trapped gas molecules [35]. Approximately 10% of the initial capacity is irreversibly consumed in SEI formation [33]. Ideally, once the SEI is created, the graphite electrode is fully coated and the reaction cannot continue. The SEI protects the anode from further reacting with the solvent, is electrically insulating, and has high selective permeability for lithium ions. A robust SEI layer is critical to good battery performance. However, SEI growth is difficult to control because it is highly dependent on the type of graphite, graphite morphology, electrolyte composition and concentration, electrochemical conditions, and cell temperature [33]. The SEI slowly corrodes with time. SEI dissolution exposes the graphite to the electrolyte, leading to additional SEI growth and thus additional capacity loss [34]. Increased temperature increases the dissolution rate, and at high temperatures, increasing voltage becomes a significant factor as well [36]. The ideal SEI is only permeable for Li+ cations, however anions, electrons, solvated cations, solvents, and impurities can diffuse through the SEI to the electrode [31]. This can result in solvent co-intercalation, creating mechanical stress within the electrode lattice. Also, electrolyte reduction within the electrode can create gases which will increase pressure and stress [31,32]. When the battery is cycled and the graphite structure is lithiated and de-lithiated, its volume expands and contracts by approximately 10% [37]. The mechanical stresses created by each of these mechanisms can lead to graphite exfoliation via particle cracking. This will decrease the amount of available active material, as well as creating additional sites for SEI growth. Lastly, these stresses can fracture and isolate electrode particles from the bulk of the material, further reducing the available active material. When a battery is at a high SoC, the anode is highly lithiated and the potential at the anode is low [38].
Tab. 1
If the potential at the anode surface is below 0 V vs Li/Li+, lithium deposition on the anode becomes thermodynamically possible. At such potentials, some lithium ions will be deposited on the surface of the electrode as metallic lithium rather than intercalated into the anode during charging [39]. To help prevent such lithium deposition, batteries are typically designed with 10% higher anode capacity than cathode capacity (N/P ratio > 1.1), so the anode is never fully lithiated [39]. Despite this precaution, lithium plating from overcharge can still occur if the initial mass ratio of lithium is higher than expected (N/P lower than expected), or if the initial LLI due to SEI growth was smaller than expected [11]. Even with properly designed ratios, high charge rates can induce lithium plating if the charge rate is greater than the rate of lithium diffusion into the graphite [29]. Low temperatures slow ion diffusion in the anode and/or the electrolyte, allowing more lithium plating and dendrite growth to occur [31]. Deposited lithium forms its own SEI layer, leading to further LLI and increased internal resistance [28,40–42]. When lithium ions are de-intercaled during discharge, metallic lithium is stripped from the anode. If electrical contact between lithium and the anode is lost, this lithium becomes “dead lithium” and is a source of capacity loss [39]. SEI can form on this dead lithium, which is an additional capacity loss mechanism [39].

2.2.2 Cathode degradation

There is greater variation in cathode degradation, since cathode aging is highly material dependent and there is a wider variety of cathode chemistries currently in use [31]. Major cathode degradation mechanisms include loss of active material and SEI growth. Loss of active material can occur when transition metals (Ni, Mn, Co, Fe) in the cathode dissolve in the electrolyte [43], in a process aptly named transition metal dissolution (TMD). This is accelerated at high temperatures. Finally, TMD can occur when the electrode is fully discharged, most significantly for cathodes containing manganese [31]. The dissolved transition metals can then deposit on the anode SEI, increasing conductivity and leading to additional SEI growth [42,43], as well as forming dendrites and decreasing the available active cathode material [31]. Like the anode, the cathode has an SEI layer, though it is much smaller than the anode layer due to the high voltage at the cathode, and is harder to measure and characterize [44,45]. Exposing the cathode to the electrolyte results in loss of lithium inventory as the cathode and electrolyte react. Lithiation and delithiation lead to volume changes and mechanical stress, which can cause cracking, creating additional reaction sites. Unlike the anode, inhomogeneous lithiation can also induce structural phase transitions in the cathode structure, such as Jahn-Teller distortion, further reducing the amount of lithium ions the cathode can accept [31]. Low state of charge (SoC) can increase this effect, but various dopants can be used to stabilize the structure [31]. These structural changes can decrease the available active material in the cathode, as well as expose the cathode to the electrolyte. Cracking can also be caused by gas generation. This can come from oxygen loss from the metal oxide at high temperatures, or from electrolyte decomposition at high voltages [11,42]. Overcharge can also cause point defects in the lattice where oxygen or transition metals take the spaces in the structure where lithium would otherwise be intercalated [46].

2.2.3 Higher order degradation

All the degradation mechanisms mentioned in Sections 2.2.1 occur at the individual cell level. There are also degradation mechanisms external to the cell that could affect the terminals or casing. Additionally, for any battery with more than one cell, there are battery pack dynamics and pack level degradation to account for. If cells in a module or pack are not balanced, they are vulnerable to overcharge, overdischarge, and overheating [48]. Active and passive balancing techniques are used by the BMS, yet as shown by Zheng et al., pack capacity will always fade more critically than cell capacity. Battery packs therefore always have a shorter lifetime than their individual cells [49]. This is primarily explained by unavoidable differences between cells due to inconsistent manufacturing or different operating and environmental conditions [49].

3 Key degradation variables

The aging process can lead to increased self-discharge rate and resistance as well as reduced capacity [28,50]. The various degradation mechanisms cited in Section 2 depend on complex and interacting mechanisms relating to cell chemistry and storage as well as charging and discharging conditions such as temperature, cycle depth, frequency of cycling, change in state of charge (ΔSoC), charge and discharge current magnitude, and elevated voltage exposure [27,51]. Battery degradation has a large impact on product performance. In EVs, for example, capacity fade influences range capability and fuel consumption, while power fade impacts driving performance, including acceleration, gradeability, and maximum charging rate during regenerative braking or charging [52,53]. In addition to factors such as the temperature distribution within the battery, DoD, SoC, and driving and charging conditions, the user's demands for power and energy also determine the operating conditions of the battery and the stress factors that influence the rate of aging [52,54]. The variables impacting degradation can be put into three main categories: temperature, state of charge, and current (C-rate) [55].

3.1 Temperature

Many studies have demonstrated the impact of temperature on LIBs both in storage and while in use. In an examination of two LFP batteries, Dubarry et al. show that the resistance of a battery tested at 60 °C was five times greater than the battery operated at 25 °C [56]. Hannan et al. argue that LIBs should be charged between 15 °C and 50 °C [41]. In another study, Pesaran et al. define 15–35 °C as the desired operating temperature for LIBs in PHEVs. They also show that lower battery degradation rate enables a smaller and lower cost battery [57]. Smith et al. apply a semi empirical model of NCA/graphite chemistry in PHEVs to investigate calendar aging in various environments with different ambient temperatures and solar radiation [58]. Their modeling shows a two year difference in battery lifetime between the ambient temperature model and ambient plus solar radiation model in Phoenix, AZ, showing the large impact of parking in the sun or the shade. Serrao et al. show that temperatures above 25 °C accelerated battery aging in Hybrid Electric Vehicles (HEVs) [59]. Hatzell et al. conclude that temperatures below -30 °C led to considerably increased cell impedance, temperatures above 60 °C led to severe capacity loss, and at temperatures above 85 °C the SEI layer decomposed, which can cause rapid degradation and thermal runaway [60]. Ramadass et al. cycled Sony 18650 LCO cells, revealing that cells at 25 °C and 45 °C lost about 31% and 36% of their initial capacity after 800 cycles, while cells at 50 °C lost more than 60% capacity after 600 cycles and cells at 55 °C lost 70% after 500 cycles [61]. Ren et al. show that the temperature at which thermal runaway begins also varies with the battery cell configuration and pressure relief design [62].
Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage. A battery dwelling above 30°C (86°F) is considered elevated temperature and for most Li-ion a voltage above 4.10V/cell is deemed as high voltage. Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling.

3.2 State of charge

Overcharge, overdischarge, and high depth of discharge lead to the fast decay of battery life [28,50,63]. Overcharge is one of the most serious problems, and can result in thermal runaway because external energy is being directly added into the battery. Dr. Dahn stresses that a voltage above 4.10V/cell at elevated temperature causes this, a demise that can be more harmful than cycling a battery. The longer the battery stays in a high voltage, the faster the degradation occurs. On the other hand, overdischarged cells experience irreversible capacity loss and changes in stability, which can affect tolerance to abuse conditions and increase the likelihood of safety issues [50]. Also, the coupling of high SoC and high temperature accelerates degradation [27]. Faria et al. recommend a cool environment with SoC around 40% to reduce the calendar aging during a long storage period [28]. They also argue that partial discharge cycles result in lower capacity loss than full discharge cycles. Capacity fade in LIBs as a result of cycling resembles the fatigue of materials subjected to cyclic loading. The accumulated stress of each cycle contributes to the loss of battery lifetime [64]. Zhang et al. show that a typical laptop battery stored at 25 °C and 100% SoC will irreversibly lose 20% of its capacity each year [15]. Ortega-Vazquez shows that the impact of cycling characteristics also depends on battery chemistry. For example, the capacity of LFP batteries is sensitive to the total number of cycles that the battery undergoes, while NCA batteries are sensitive to the total number of cycles and to the DoD of the cycles [65]. Amiri et al. conclude that smaller changes in SoC during cycling increases battery lifetime [66]. Millner specifies that the battery lifetime can be kept in an acceptable range for Plug-in Hybrid Electric Vehicles (PHEVs) by avoiding deep cycles (>60% DoD), high temperatures (>35 °C), and high average SoC(>60%) [67]. Marano and Madella show that to reach 10 year/150,000 mile PHEV lifetime, overcharging and operation above 95% SoC should be avoided. They also show efficiency and performance degradation if LIBs are discharged or operated at lower than ~25% SoC [54]. Hoke et al. argue that if battery temperature and charge-discharge cycling are kept constant, minimizing time spent at high SoC minimizes degradation [68]. If the next day's energy requirement is known, the battery can be charged to the minimum required level, rather than to the conventional full charge [68]. Trippe et al. define 60% to 97% SoC as the safe window to preserve battery health [40]. Lunz et al. show that battery lifetime can be increased by reducing the target SoC to lower values, or by minimizing rest periods at high SoC. Therefore, battery charging should occur immediately before departure [69]. Because standby times dominate battery operation, there is a large opportunity to increase battery lifetime by adjusting the time and frequency of charging (smart charging). For PHEV batteries, Smith et al. suggest several strategies to reduce calendar aging from high SoC. These include reducing time spent at high SoC by just-in-time (delayed) charging, and intentional partialdepletion of the battery from vehicles parked in hot environments (e.g., by running the cooling system) until an appropriate SoC is reached [58].
Similar to an EV, Li-ion in satellites must also endure a lifespan of 8 years and more. To achieve this, the cells are charged to only 3.90V/cell and lower. An interesting discovery was made by NASA in that Li-ion dwelling above 4.10V/cell tend to decompose due to electrolyte oxidation on the cathode, while those charged to lower voltages lose capacity due to the SEI buildup on the anode.
NASA reports that once Li-ion passes the 8 year mark after having delivered about 40,000 cycles in a satellite, cell deterioration caused by this phenomenon progresses quickly. Charging to 3.92V/cell appears to provide the best compromise in term of maximum longevity, but this reduces the capacity to only about 60 percent.
Most chargers for mobile phones, laptops, tablets and digital cameras charge Li-ion to 4.20V/cell. This allows maximum capacity, because the consumer wants nothing less than optimal runtime. Industry, on the other hand, is more concerned about longevity and may choose lower voltage thresholds. Satellites and electric vehicles are such examples.
For safety reasons, many lithium-ions cannot exceed 4.20V/cell. (Some NMC are the exception.) While a higher voltage boosts capacity, exceeding the voltage shortens service life and compromises safety. Fig. 3 demonstrates cycle count as a function of charge voltage. At 4.35V, the cycle count of a regular Li-ion is cut in half.

Fig. 3: Effects on cycle life at elevated charge voltages. Higher charge voltages boost capacity but lowers cycle life and compromises safety

Besides selecting the best-suited voltage thresholds for a given application, a regular Li-ion should not remain at the high-voltage ceiling of 4.20V/cell for an extended time. The Li-ion charger should turns off the charge current and the battery voltage reverts to a more natural level. This is like relaxing the muscles after a strenuous exercise.

3.3 C-rate

In addition to temperature and DoD/SoC, battery aging also depends on accumulated charge transfer in and out of the battery (amp-hour throughput), and the current magnitude relative to battery size (C-rate) [54]. Higher charging and discharging current rates can accelerate cell degradation due to an uneven distribution of current, temperature, and material stress, where Li-ion intercalation and diffusion speed are the limiting factors. These unevenly distributed conditions can lead to uneven ageing, including deposition of metallic lithium, and SEI growth at certain parts of the electrodes [70]. High-rate discharge means a short period of time for Li-ion transfer. In such conditions, ions are not fully de-intercalated, which results in capacity fade and lithium dendrite formation. Higher current rates also lead to higher internal temperature, encouraging side reactions that increase the loss of active material. There is always capacity fade and accelerated aging during high-rate discharge [50]. In an experimental study with post-mortem analyses, Mussa et al. show that the dominant degradation mechanisms may depend on C-rates [71]. For example, 3C charging resulted in additional lithium plating, while 4C charging resulting in graphite exfoliation and gas evolution [71]. Wang et al. note that different charging protocols perform best at different cycling temperatures, and that there is no one ideal charging protocol for all batteries [72]. Illustrating some of these degradation mechanisms, Fig. 4 shows that lithium ions are able to diffuse homogeneously throughout the electrode lattice at low current. With high charging current, the ion diffusion rate is slower than the charging rate, leading to an inhomogeneous distribution of ions throughout the lattice. This can cause lithium plating on the surface of the electrode, as well as stress-induced cracking and loss of active material.

Fig. 4 demonstrates capacity loss caused by the structural degradation of a Li-ion when cycled at a 1C, 2C and 3C. The elevated capacity loss at higher C-rates may be lithium plating at the anode
caused by rapid charging.

4 Comparing manufacturer instructions and academic literature

The different audiences for academic literature and manufacturer instructions necessitate differences in how information is presented. While academic studies often give very specific insights about battery performance in response to one or occasionally two key variables, manufacturer instructions give broad and actionable information to users. Despite the differences in granularity and specificity of the information presented, the underlying information given should be the same. However, we have found that this is not true in all cases. In Fig. 6, variables affecting battery degradation, identified from both academic literature and manufacturer guidance, are compared with the percentage of companies making a recommendation related to that variable. Fig. 6a shows the comparison in total, while Fig. 6b - e shows industry recommendations for each device type.

Fig. 6: Percentage of surveyed companies warning users against exposure to certain conditions for a) all companies surveyed, and b–e) by manufacturer of each device type surveyed

No EV companies recommend against keeping the battery at 100% state of charge. This is because keeping the vehicle plugged in allows the BMS to control battery temperature using power from the grid, which is deemed more important, as well as potentially more palatable, than telling users to leave their vehicle less than fully charged.
Cellphones (Fig. 6b) include fewer warnings against high and low state of charge than laptops (Fig. 6c). This may be because users often replace cellphones before the degradation
becomes significant (replacement cycle length of 2.8 years), while users expect longer lifetimes from their laptops (replacement cycle length of 6.9 years in the US) [135].

5 Battery lifetime improvement

Maximizing battery lifetime has environmental and economic benefits; but to maximize lifetime, one must avoid storage and use conditions that accelerate degradation. Avoiding these adverse conditions is the responsibility of a device's BMS and user actions.

5.1 Benefits of battery lifetime improvement

The environmental benefit of LIB lifetime extension is due to reduced demand for and production of new and replacement batteries. For example, manufacturing a single Dell laptop battery (<1 kg) results in 10 kg of CO2e emissions [136]. In general, manufacturing has a dominant share in CO2e emissions of average cellphones, tablets, and laptops [137]. A report by Green Alliance claims that extending the lifetime of a cellphone by 1 year reduces by 1/3 the lifetime CO2e impact of the device [137]. Along with energy use and resulting emissions, battery production also contributes to ozone depletion, photo oxidation formation, particulate matter formation, terrestrial, freshwater, and marine eutrophication, freshwater and marine toxicity, terrestrial acidification, and the human health impacts of each of these [138]. In addition to environmental benefits, there are clear economic incentives for users to extend battery lifetime. For Apple devices, battery replacement cost (out of warranty) is a substantial percentage of total device cost, at 5%–9% for phones [139], 12%–30% for tablets [140], and 7%-15% for laptops [141]. For power tools this can be even more extreme. Depending on the battery and tool, a battery could cost twice as much as the tool itself [142–144]. And a BEV battery pack represents 35%-50% of the total price of the vehicle [145]. Though EV battery costs have fallen dramatically in recent years, the U.S. Department of Energy goal of $125/kWh production cost by 2020, if met, results in a production cost of $7,500 for a 60 kWh battery to $12,500 for a 100 kWh battery, which remains a substantial percentage of vehicle cost [146].

5.2 User behaviors

Based on the academic literature and information provided in owner's manuals, user guides, and customer support websites, a list of behaviors was developed to illustrate nine best practices for maximizing Li-ion battery lifetime, shown in Table 4 and explained in more detail below. These best practices are general in nature, and written for the end users of products with LIBs. Every practice will not apply to every battery, as operating requirements and the role of the BMS vary between devices.

5.2.1 Temperature recommendations

Elevated temperatures can accelerate degradation in almost every component of LIBs. This impact is greatest when combined with high voltages, but can occur regardless of the SoC [27]. Furthermore, elevated temperatures can lead to significant safety risks, as gas may form within the battery increasing pressure to the point of explosion. Recommended high temperature limits are stricter when in use than when in storage. Typically, if a device is noticeably hot when charging, it should be unplugged. However, most EV manufacturers recommend that vehicles should be plugged in when the ambient temperature is hot, so the vehicle's battery cooling system can operate directly from grid electricity. When a battery is cycled at low temperatures it is more susceptible to lithium plating, which can lead to internal short circuits irreparably damaging the battery, and potentially causing safety issues. For power tools and EVs, chargers will not begin charging until the device has reached an appropriate temperature, and for EVs this may include the use of a battery heating system.

5.2.2 State of charge recommendations

There are two main strategies to minimize time spent at 100% SoC. First, devices can be partially charged, unless a full charge is needed. Second, devices should be unplugged once they reach 100% SoC.
Most Li-ions charge to 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life. For example, a lithium-ion cell charged to 4.20V/cell typically delivers 300–500 cycles. If charged to only 4.10V/cell, the life can be prolonged to 600–1,000 cycles; 4.0V/cell should deliver 1,200–2,000 and 3.90V/cell should provide 2,400–4,000 cycles.
On the negative side, a lower peak charge voltage reduces the capacity the battery stores. As a simple guideline, every 70mV reduction in charge voltage lowers the overall capacity by 10 percent.
In terms of longevity, the optimal charge voltage is 3.92V/cell. Battery experts believe that this threshold eliminates all voltage-related stresses.
Just as high SoC places stress on a battery, so too does low SoC. A device's BMS will shut down a device before it reaches true 0%, to avoid overdischarge, which can permanently damage the battery.

Tab. 2: Discharge cycles and capacity as a function of charge voltage limit

Every 0.10V drop below 4.20V/cell doubles the cycle but holds less capacity. Raising the voltage above 4.20V/cell would shorten the life. The readings reflect regular Li-ion charging to 4.20V/cell.

Guideline: Every 70mV drop in charge voltage lowers the usable capacity by about 10%.
Note: Partial charging negates the benefit of Li-ion in terms of high specific energy.
* Similar life cycles apply for batteries with different voltage levels on full charge.
** Based on a new battery with 100% capacity when charged to the full voltage.
Experiment: Chalmers University of Technology, Sweden, reports that using a reduced charge level of 50% SOC increases the lifetime expectancy of the vehicle Li-ion battery by 44–130%.

5.2.3 Current recommendations

Fast charging is convenient, but it comes with a trade-off. Repeated use of fast chargers will degrade a battery more quickly than standard charging. Discharging a battery too quickly leads to battery degradation through many of the same mechanisms as fast charging. One way to determine if a battery is discharging too quickly is if it is noticeably hot. Discharging currents can be controlled by the user to various degrees depending on the device. For cellphones and laptops, lowering screen brightness, turning off location services, and quitting high power using applications can help. For power tools, choosing a tool with sufficient power output for the task at hand is important. And for EVs, driving habits, such as limiting sudden starts and stops, will impact the battery pack's discharging current.

5.2.4 Charging Cobalt-blended Li-ion

Li-ion with the traditional cathode materials of cobalt, nickel, manganese and aluminum typically charge to 4.20V/cell. The tolerance is +/–50mV/cell. Some nickel-based varieties charge to 4.10V/cell; high capacity Li-ion may go to 4.30V/cell and higher. Boosting the voltage increases capacity, but going beyond specification stresses the battery and compromises safety. Protection circuits built into the pack do not allow exceeding the set voltage.
Figure 7 shows the voltage and current signature as lithium-ion passes through the stages for constant current and topping charge. Full charge is reached when the current decreases to between 3 and 5 percent of the Ah rating.

Fig. 7: Charge stages of lithium-ion
Li-ion is fully charged when the current drops to a set level. In lieu of trickle charge, chargers apply a topping charge when the voltage drops.

When the battery is first put on charge, the voltage shoots up quickly. This behavior can be compared to lifting a weight with a rubber band, causing a lag. The capacity will eventually catch up when the battery is almost fully charged (Figure 8). This charge characteristic is typical of all batteries. The higher the charge current is, the larger the rubber-band effect will be. Cold temperatures or charging a cell with high internal resistance amplifies the effect.
Li-ion is fully charged when the current drops to a set level. In lieu of trickle charge, chargers apply a topping charge when the voltage drops.
When the battery is first put on charge, the voltage shoots up quickly. This behavior can be compared to lifting a weight with a rubber band, causing a lag. The capacity will eventually catch up when the battery is almost fully charged (Figure 8). This charge characteristic is typical of all batteries. The higher the charge current is, the larger the rubber-band effect will be. Cold temperatures or charging a cell with high internal resistance amplifies the effect.

6 Conclusion

Lithium-ion batteries inevitably degrade with time and use. Almost every component of the battery is affected, including the anode, cathode, electrolyte, separator, and current collectors. A wide variety of mechanisms contribute to degradation, and these mechanisms are sensitive to storage conditions and use patterns, including temperature, SoC, and charging/discharging rate. By minimizing exposure to the conditions that accelerate degradation, batteries can last longer. This has a positive environmental impact, as battery production is a source of GHG emissions and many other pollutants. Additionally, there are significant financial incentives for users to avoid adverse conditions, as the cost of batteries can range from 5% to over 50% of a product's total cost. Despite these clear benefits, user understanding of proper battery management is lacking and guidance provided through product manuals and company websites often is scattered, contradictory, or non-existent. Additionally, there is a significant lack of knowledge regarding how users operate batteries. Limited research has shown user knowledge of battery health issues to be poor, but further survey data are needed to establish actual battery use patterns to quantify the net impact of user behavior on battery lifetime.


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