Johnson Matthey Technol. Rev., 2020, 64, (3), 287
Battery Materials Technology Trends and Market Drivers for Automotive Applications
Challenges for science and industry in electric vehicles growth
With the electric vehicle (EV) market set to grow rapidly over the coming years, the industry faces a challenging ramp-up of volume and material performance demands. From the current trend towards high-energy high-nickel cathode materials, driven in-part by consumer range anxiety, to the emergence of solid-state and beyond lithium-ion technologies, herein we review the changing requirements for active materials in automotive Li-ion battery (LIB) applications, and how science and industry are set to respond.
Outdoor air pollution is linked to an estimated 4.2 million deaths each year worldwide (1). Tailpipe emissions from conventional internal combustion engine (ICE) vehicles are a major contributor to urban air pollution, and as such have been subject to ever tighter legislation for decades, requiring increasingly innovative improvements and catalytic emissions controls. We have now reached the point where a move away from the ICE is required to continue air quality improvements, with several countries going so far as banning new purely ICE vehicles in the coming years. This is where EVs will play their part – both pure EV and hybrid systems powered by LIB technologies, as well as fuel cell technologies, are set to see increased uptake and demand as we strive for cleaner air. In this article, we will add to the automotive-focused literature (2–4) and review what technologies are required to drive the uptake of pure EVs, and what the industry is doing now to respond to consumer requirements as this market rapidly grows.
There are several characteristic battery parameters that it is important to consider and contrast with consumer behaviours and expectations for automotive applications: perhaps most significant, the energy or capacity of the cell equates to the ‘miles in your tank’, and is an area where EVs have lagged behind the ICE in previous years. This is evolving, with the most successful EVs on the market now having an average range of 350 km (5). Range anxiety, equating to energy density, is a major theme for the battery materials industry, with contributions from and innovations required in three areas: the cathode, anode and electrolyte. Cost is also an important factor; as well as the material costs for the active components, analysis has shown that the electrode thickness within the cell is a major contributor to automotive cell costs (6) – materials with increased volumetric energy density are therefore additionally attractive from this perspective. There is also the practical cost benefit afforded by developing systems that can operate at higher voltage cut-offs (7), owing to the usable advantages, towards which multiple cell components can be developed and optimised. Herein, we review one topic of significant industry focus from each area: high-Ni cathode materials, with lithium nickel manganese cobalt oxide (NMC) 811 and beyond being commercialised within the next three years; high energy silicon anode technologies, expected to be at commercial scale in the next three to five years; and solid-state electrolytes, with significant progress expected from the next five years and beyond.
High Energy Cathode Advancements
Whilst the cathode active material technology landscape remains diverse, with no one material that will meet all EV requirements, the general trend for passenger EVs is using high-Ni NMC, and lithium nickel cobalt aluminium oxide (NCA) materials. The layered Li Ni oxide (LNO), has been studied for the past 25 years, ever since the commercial application of the isostructural Li Co oxide (LCO) by Sony, Japan, in 1991; the relative low cost of Ni compared to Co was an initial driver for this work – and continues to be a factor today (8–12). Until relatively recently, automotive industry uptake was focused on lower Ni NMC variants, such as LiNi1/3Mn1/3Co1/3O2 (NMC 111), and lower energy chemistries such as Li Mn oxide (LMO), and Li iron phosphate (LFP). Tesla, USA, bucked the trend; as an early adopter of higher‐Ni NCA materials, it was ahead in the EV mileage stakes. Now, driven by consumer demand for more range, high-Ni is in vogue – the key for research and industry alike is to innovate-out the technical problems associated with LNO regarding its stability.
LNO tends towards non-stoichiometry, owing to the relative instability of Ni3+ compared to Ni2+, and the similar ionic radii of Ni2+ (0.69 Å) and Li+ (0.73 Å) (12, 13). It has been shown that synthesis conditions are key to prevent the formation of Ni2+ anti-site defects, with near-stoichiometric LNO requiring control of calcination temperature, atmosphere and Li content (12, 14). LNO is also known to undergo several phase transformations on electrochemical cycling; whilst a capacity of over 200 mAh g–1 can be achieved, these transformations lead to significant capacity fade over the first cycles (15). Early research showed the benefits of incorporating relatively small amounts of other metals, most notably Co, Al and Mn, into the structure to impart stability and significantly improve capacity retention. Owing to the isostructural nature of its end members, all compositions in the series LiNi1–x Cox O2 (x = 0–1) can be formed; Co3+ imparts stability by hindering the formation of Ni2+ anti-site defects (16). Conversely, doping Mn into the LNO structure has been shown to detrimentally effect the reversible capacity but to impart thermal stability benefits – a key property for battery safety (17, 18). The beneficial effect of Al substitution at low levels is two-fold: an improvement in capacity retention by minimising detrimental phase transformations and an increase in thermal stability (18, 19). There is, however, a limitation to the amount of Al that can be usefully incorporated into the structure; the addition of high-levels of an electrochemically inactive dopant will result in a reduction in capacity, and Al3+ has been shown to segregate and create localised defects within the lattice, due to the different ionicity of Al–O and Ni–O bonds (20).
This combined work has ultimately led to continued focus on the multiple metal dopant strategies found in NCA and NMC, where greater benefits are observed than in single dopant systems. Whilst not as catastrophic as those in LNO, NCA and high-Ni NMC materials (such as NMC 811) undergo significant structural changes on cycling, which their lower Ni counterparts (for example NMC 622, NMC 111) do not (Figure 1): at high states of charge, a transformation from the second hexagonal phase (H2) to the third hexagonal phase (H3) occurs in high-Ni materials that is associated with c lattice contraction and capacity fade (21–23). The addition of dopants to the bulk structure of LNO such as cobalt, manganese, aluminium, magnesium, titanium and combinations thereof has been shown to influence stability by affecting the volume change on cycling associated with the H2/H3 phase transformation (24–26).
Coating strategies have been employed to high‐Ni NCA and NMC systems, providing benefits in two key areas: handling and performance. The handling and processability of high-Ni materials is a well-known challenge, with surface reactivity towards the ambient resulting in the formation of Li hydroxide and Li carbonate impurities, and the resultant propensity of electrode slurries to gel: this creates obvious challenges before materials have even reached the cell (27–29). Once in the cell, these surface impurities contribute to resistance growth and side reactions resulting in gassing (30, 31). Moreover, the high-Ni surface itself is known to undergo phase changes upon cycling, with the formation of the rock salt phase Ni oxide also contributing to instability and capacity fade (32, 33). In its simplest sense, the application of an inactive coating such as Al oxide passivates the surface with respect to these undesirable side reactions, creating more benign materials that are easier to handle; but only so much of this type of coating can be applied before either significant capacity loss or resistance gains are observed (34). As such, the move toward active coatings, where the removal of an inherent risk of capacity loss does not limit the amount or depth of coating that can be applied, is very attractive. A notable example in this area is the extensive work by the Sun group, who have developed several generations of active coatings and complex morphologies for high-Ni materials (Figure 2): starting with a core@shell strategy, a low-Ni NMC was applied to the surface of a high-Ni NMC, creating a system that combined a high-energy core with a high-stability surface and building a system that was electrochemically active throughout (35). The drawback of this system was the observation that the shell layer broke away from the core on cycling, due to the mismatched volume changes within the core and shell NMC layers. To counteract this, the group developed a gradient coating strategy, whereby a lattice expansion or contraction mismatch was avoided by creating a continuous region of gradual compositional change, thus removing a core@shell interface (38). The Sun group further extended this work to look at deeper and multi-component gradients and their potential benefits (36, 37, 39). Such gradient systems can be viewed as a sophisticated hybrid between bulk doping and surface coating strategies, helping to mitigate the trade-offs associated with each strategy alone.
These gradient systems demonstrate the importance of considering morphology and process alongside composition in materials engineering. Another area of interest is the mitigation of microcrack formation through the control of primary particle shape, size and interfaces; fewer cracks means a more stable cathode electrolyte interface (CEI) layer, alleviating resistance growth and gas-generating side reactions (33, 40, 41). Most recently, this has led to particular interest in single crystalline morphologies, which promise greater long-term cycling stability compared to their polycrystalline counterparts by minimising the number of interfaces where microcracks can occur. The majority of published research in this area has focused on lower-Ni NMCs (i.e. NMC 622 or less), where reduction in gassing has been observed compared to polycrystalline counterparts, albeit at the cost of rate capability (42, 43). This lower Ni focus is in part due to the challenging nature of high Ni synthesis at the typically elevated temperatures required to form single crystalline materials compared to those used to generate polycrystalline materials. There are examples demonstrating similar advantages for a single crystalline morphology with up to 80% Ni content and efforts are clearly growing in this area: single crystalline NMC 811 has been shown to exhibit less gassing than its polycrystalline counterpart during high temperature storage (30). Zhu et al. undertook a broad study looking at NMCs from NMC 111 to NMC 811 prepared by multiple approaches and demonstrated the need to tune synthesis conditions to Ni content (44).
The engineering opportunities to overcome the challenges presented by high Ni materials continue to grow. As the automotive industry strives for higher energy, the drive to increase the Ni content of NCA and NMC type materials is clear – the common theme across the industry is to move from NMC 622 to NMC 811 and toward 90% Ni content to meet energy requirements, but also to reduce the Co content required, due to sourcing and cost challenges. Ultimately, a combination of the strategies reviewed above are required to develop and commercialise materials with a Ni content of 80% and above to meet the energy and stability requirements of the automotive industry.
High Energy Anode Advancements
Aligned with the drive toward higher energy cathode materials, there is a requirement to enhance and optimise LIB anode materials toward greater energy density, improved cycle life, lower cost per kilowatt hour and improved gravimetric and volumetric densities (3, 46). In particular, the use of higher energy cathode materials allows increased ampere hour per geometric area and volume of active cathode which is important to retain realistic active material loadings and thicknesses and achieve battery EV (BEV) cell and pack targets. A commensurate improvement in storable energy per area and volume of anode electrode is therefore also required. Cell manufacturers and original equipment manufacturers (OEMs) are increasingly moving beyond todays natural and synthetic graphite materials (or combinations of these) toward blending graphite with a higher energy density Si or Si oxide component to enhance cell level energy gravimetric and volumetric density (47). Table I illustrates examples of such Si containing materials (48).
|Volume change % during lithiation||12||280||160|
|Lithiated phase||LiC6||Li15Si4||Lix Si, Li2O, Li4SiO4|
|Initial theoretical specific capacity, mAh g–1||372||3579||3172|
|Typical initial coulombic efficiency, %||90–95||77.5–84||65–95|
aReproduced from Chen et al. and references therein (48)
The high natural abundance of Si and low operating voltage (0.2 V discharging potential compared to Li/Li+) single out Si as a highly promising anode material for LIBs (49). However, Si containing materials as battery anodes exhibit a number of challenges, with the greatest of these being significant volume expansion during the lithiation process (see Table I). Particle cracking or fragmentation, loss of electrical contact, ongoing parasitic reactions between electrolyte and ‘fresh’ surfaces, cell swelling and gassing all contribute to cycle life issues (see Figure 3 and Figure 4) (46). Various approaches can be deployed to address the volume change issue for pure Si anodes, including nano-engineering of the Si electrode structure (nanowires and nanoparticles, formation of secondary agglomerates) along with advanced binder combinations to create a flexible electrode structure (46, 50, 51). The addition of carbon dioxide into pouch cells has also been trialled to limit parasitic reactions (52). Formation of nanocomposites of Si–C via mechanical or chemical deposition processes, addition of other alloying components or the choice of a SiOx material (where first cycle lithiation allows an irreversible reaction creating stabilising LiOx and Li silicate components within the structure) can all bring improvements (50, 53). Incorporation of conductive carbon also addresses the challenge posed by the intrinsic low conductivity of Si containing materials (54).
A strategy of blending Si containing materials with existing graphite types is already in progress to achieve moderate capacity increase and lessen volume change, as illustrated by cell level calculations for this approach (for example Si:C 1:3 with capacity of 1100 mAh g–1 by Andre et al.) (3, 47). Table I illustrates an additional challenge present in Si containing anodes in the form of lower first cycle efficiency (FCE) vs. graphite, related to reactions consuming Li between the electrolyte and anode, the formation of the SEI and associated reduction in useful Li inventory in the working cell, reducing effective watt hour per kilogram. Pre-lithiation approaches, where sacrificial Li containing materials are added to the Si anode during electrode fabrication or strategies such as electrochemical pre-lithiation of formed electrodes ahead of cell assembly are possible (55, 56) along with chemical pretreatments ‘artificial SEI formation’ (57, 58). However, these all represent additional steps and cost in a cell manufacturing process, also pre-lithiated materials and electrodes and Si nanoparticles require careful handling due to the reactivity of the materials with moisture and air (48).
Careful optimisation of the liquid electrolyte additives is also crucial to achieve prolonged cycle life and good FCE, with fluorinated additives, especially fluoroethylene carbonate (FEC), showing benefit (59). The discharge and charge voltage profile of Si containing anodes is slightly different to graphite-only examples, leading to reduced chance of Li plating during charging in Si anodes, but typically slightly lower discharge voltage with graphite, thus adjustments to cell balancing and understanding of the operational state of charge window in the usable voltage range are important for full cell (60).
Assessment of the sustainability of changing to Si containing anode components and advanced higher energy cell chemistries is also vital as electrification of the power train advances worldwide (61).
Higher Energy Through Solid-State Electrolytes
A further driver to increase the energy density of cells is to replace existing anode materials with metallic Li. Li metal was used as the first anode material in rechargeable Li-ion cells due to its very high energy density (3860 mAh g–1) and low electrochemical potential (–3.040 V vs. the standard hydrogen electrode). However, numerous challenges prevented its widespread adoption, including low cycle life predominating from issues such as the formation of dendrites and unstable solid-electrolyte interfaces. Recently, there has been increasing investigations into using solid-state electrolytes to mitigate the challenges of using metal anodes, whilst maintaining their advantages.
In addition to potentially enabling the use of Li metal anodes, the evolution to solid state batteries has other advantages to conventional Li-ion cells (62). The primary reason is the displacement of the highly flammable cocktail of organic electrolytes that is used currently. This both reduces the risk of unwanted thermal events in the instance of cell misuse or damage, but it also results in a simpler packaging, further increasing the energy density (63) (Figure 5). In addition, solid state materials could offer increased electrochemical stability windows in comparison to existing organic electrolytes; potentially enabling alternative materials, such as higher voltage cathode materials, to be deployed.
The use of polymers as electrolytes in batteries was first pioneered in the 1970s (64, 66). This enables cells with high degrees of safety to be manufactured in various form factors. Polymer-based systems such as polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), polyacrylonitrile and polymethyl methacrylate (PMMA) based electrodes have all been widely studied as polymer electrolytes (67). PEO-based polymer electrolytes have been studied the most due to their advantageous properties including lower cost, ability to solvate a wide variety of ions, relatively high chemical stability and the use of their moderate mechanical strength (~106 Pa) to supress the growth of dendrites (68, 69). However, the low conductivity (~10–7 S cm–1) of the electrolyte systems, due to the crystallinity of the polymer chains, has been a limitation (70). Overall, the general uptake of polymer gel cells has been restricted by their lower energy densities and poor electrochemical stability compared to liquid electrolytes.
All Solid-State Batteries
More recently, researchers have explored a range of solid inorganic materials, which allow ionic mobility through the solid. Numerous classes of these are currently being explored, all possessing different advantages and disadvantages (63, 71, 72). A summary of these are highlighted in Table II.
|Type||Example composition||Ionic conductivity at room temperature (RT), S cm–1||Electrochemical stability to Li|
|Sulfide||Li10GeP2S12 (73)||1 × 10–2||Stable|
|Garnet||Li7La3Zr2O12 (74)||3 × 10–4||Stable|
|Sodium superionic conductor (NASICON)||Li1.3Al0.3Ti1.7(PO4)3 (75)||7 × 10–4||Unstable|
|Perovskite||Li0.34La0.51TiO2.94 (76)||2 × 10–5||Unstable|
|Lithium phosphorous oxynitride (LiPON)||LiPON (77)||6 × 10–6||Stable|
|Anti-Perovskite||Li3OCl (78, 79)||9 × 10–4||Stable|
|Argyrodite||Li6PS5Cl (80)||1 × 10–3||Stable|
Researchers have looked to examine inorganic electrolyte materials with high ionic conductivities, such as Li10GeP2S12, which exhibits high conductivity at RT (73). However, sulfide-based solid electrolytes are generally expensive, more challenging to synthesise and are sensitive to moisture, potentially releasing toxic gases. This brings challenges in their handling and subsequent fabrication.
Although most solid electrolytes have been shown to react with Li metal, garnet materials (such as Li7La3Zr2O12 (LLZO)), have shown the greatest stability (74, 75). In addition, they have relatively low costs and a wide electrochemical window (~6 V vs. Li metal) potentially enabling the use of higher voltage cathode materials; and are therefore attracting increasing investigations (74). The cubic phase of LLZO is found to offer greater ionic conductivity than the tetragonal phase. A typical strategy to promote this is to dope elements such as Al, tantalum and gallium into the structure thus stabilising the highly conductive cubic phase at RT (76).
Despite these advantages, a challenge in using LLZO remains its instability in the ambient atmosphere, due to CO2 and moisture (77). This results in increased complexity upon subsequent material handling and processing. Further challenges include poor interfacial compatibility of LLZO with electrodes. To overcome this, methods to increase the wettability of the electrolyte have been explored, such as the atomic layer deposition of Al2O3 to reduce interfacial resistance by the formation of a desirable Li-Al-O layer (73); or alloying Li with other elements (such as Si, Al, Ge) to increase compatibility (72).
In addition to the preparation of materials capable of high levels of Li-ion conductivity, it is vital that these materials can be manufactured at an industrial scale at a reasonable cost. While there has been considerable interest in the use of oxides for an all solid electrolyte, their brittleness and fragility impose new challenges for mass production (78, 80). As a result the scale up of such activities is being explored using a variety of different processing technologies (Figure 6). Mature slurry-based technologies have been shown to provide dense layers using high throughput techniques. However, subsequent high temperature sintering inhibits the co-firing of solid electrolytes and cathode particles.
When using Li metal as an anode material it is vitally important to prepare dense electrolyte layers in the absences of holes. It has been suggested that a critical relative density of >93% are required to eliminate the formation of dendrites in LLZO electrolytes (79); with short circuits believed to propagate through voids and grain boundaries (81). To obtain highly sintered garnet-based solid electrolytes by conventional sintering techniques, generally high temperatures (>1200°C) and long sintering times (>30 h) are required. Such conditions can result in the decomposition of the solid electrolytes and loss of Li from the structure.
To overcome these challenges, alternative processes such as hot pressing, field-assisted sintering and spark plasma synthesis have been investigated to fabricate the optimal dense ceramic layer (82–85). To that end further evaluation of deposition and sintering technologies will be required to provide an economically viable solution.
There are also multiple technologies (such as Li‐sulfur and Li-air chemistry) that have the potential to deliver significant advances in performance, such as increased energy density (86). For example, Li-S chemistry benefits from the low cost and high abundance of S and an energy density significantly higher than current Li-ion cells (~2500 Wh kg–1) (87, 88). However, these technologies currently suffer from technical challenges that limit their uptake. To fully maximise the benefit of these technologies, it is necessary to overcome the challenges of working with a Li metal anode. The use of solid-state electrolytes is a recent area where people have been exploring with the aim of enabling the technology via anode protection.
The demand for cleaner air is accelerating and this is giving rise to increased electrification in the automotive drivetrain. There is also a growing acceptance of vehicles with varying degrees of electrification, and this trend looks set to continue. Current concerns for increased energy density to counter consumer’s ‘range anxiety’ are leading to material developments to meet this. In particular, the careful design and manufacturing of cathode materials with high amounts of Ni and anode materials with increasing Si content are steadily improving these key parameters. Furthermore, significant exploration into next generation technologies, such as solid-state electrolytes, opens the possibility of redesigning the cell. While options to the type of material used and their processing remain; the replacement of conventional liquid electrolytes promises to deliver further improvements in energy density as well as other benefits, such as safety performance. These three examples highlight the major trends being investigated and introduced into automotive cells to meet the demands of society.
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Sarah Ball is the Applications Innovation Manager, Battery Materials, at Johnson Matthey, UK. She is the technical lead for application testing equipment specification and testing development, working across Johnson Matthey’s new battery application testing facilities in Billingham (County Durham, UK) and Milton Park (South Oxfordshire, UK). She has been involved in work on a range of beyond lithium-ion and lithium-ion batteries projects. Previously she was involved in fuel cell research on novel cathode materials including assessment of electrochemical stability, performance and properties.
Joanna Clark is Head of Product Development, Battery Materials at Johnson Matthey. She obtained her PhD from University of Liverpool, UK. She is responsible for development of new products in the battery materials research group at Johnson Matthey.
James Cookson is a Research Manager at Johnson Matthey, Sonning Common. He obtained a DPhil in Inorganic Chemistry at the University of Oxford, UK. James leads Johnson Matthey’s research activities in battery technologies; exploring both Li-ion as well as next generation technologies.