Our overall goal is to relate external stimuli and stress (high temperature, rates) to physical and chemical processes that occur in the battery, developing a mechanistic understanding of battery degradation. This will provide the automotive industry with both signatures of degradation and also predictive insight into how further degradation can be avoided through materials and systems design.
Cathode work package
Exploring the origins of slippage and impedance growth in four areas:
- Inter-granular and intra-granular particle cracking
- Surface rock-salt layer formation and growth
- Electrode-electrolyte interactions including electrolyte oxidation and double layer structure
- The role of novel coatings and dopants on mitigating one or more of these
Anode work package
- Determine how and why NMC811 cathodes trigger increased lithium inventory loss at the anode and why higher voltages trigger increased slippage
- The overall objective will be achieved by considering the chemical and mechanical stability of the solid-electrolyte interphase (SEI) – tasks are grouped into two general areas:
- Examine the role of alternative electrolytes and additives on SEI stability
- Examine effect of cross-talk mechanisms – caused by e.g., transition metal dissolution and electrolyte oxidation products – on SEI stability
- Contrast results from two different carbons to explore generality of findings
Phase 1 (2018-2021)
Work package 1: Chemical Degradation (WP1 lead: Clare Grey, Cambridge)
Chemical (i.e., molecular-level) degradation processes typically start at interfaces, of which there are many:
- the anode SEI,
- cathode electrolyte interphase (CEI),
- the interfaces between secondary (agglomerated) particles in a larger electrode,
- those between primary particles including those that form on particle cracking,
- carbon–electrode (wiring) contacts,
- between the current collector and electrode, etc.
The overarching challenge is to link a specific impedance signature to the relevant interfacial degradation process – and then to either develop a cycling regime to mitigate it or chemical solution to prevent it. These interphases are typically of limited thickness (<100 nm), consisting of multiple species, and may change when removed from the battery environment for characterisation. We have therefore developed a range of complementary operando methods to probe the chemical structure of these interphases during electrochemical cycling, complemented by and correlated with, ex-situ studies.
Work package 2: Understanding Materials Driven Degradation (WP2 lead: Paul Shearing, UCL)
Electrode performance loss is driven by a range of external stimuli including temperature, current, rate and pressure, which independently influence microstructure, local stoichiometry, crystallography and interphase structure, at both anode and cathode. This work package aims to understand the degradation of electrodes in response to a range of accelerated stress tests (ASTs) at the particle-level lengthscale. We will correlate chemistry, microstructure and mechanical behaviour of electrodes leveraging expertise and infrastructure spanning imaging and spectroscopy. The effects of temperature, current density and cycling rate, electrolyte composition, and the presence of additives on the structure of the anode and cathode are studied with operando techniques, as well as with post-mortem atomic resolution analysis.
Work package 3: Understanding Electrochemical Degradation and its Signals (WP3 lead: Ulrich Stimming, Newcastle)
In WP3 we aim to:
- Identify the electrochemical signals related to degradation in order to detect these processes as early as possible. In-situ Scanning Probe Microscopy, together with UHV techniques such as SIMS, Low energy ion scattering (LEIS), Auger spectroscopy and XPS are used in conjunction with EIS to identify degradation mechanisms.
- Develop suitable operation parameters for the cells that avoid continuation of degradation reactions.
- Transfer this understanding from the half-cell to the cell, to the module and the stack.
- To design a battery management system (BMS) that is able to detect all degradation signals for each cell in a stack with high accuracy and can initiate countermeasures.
Work package 4: Materials Design (WP4 lead: Serena Corr, Sheffield)
To identify and understand degradation processes reliably and robustly, we are developing high quality materials in a reproducible manner with precisely defined crystal chemistries for use across the WPs. To maximise our outputs over the three-year period, this WP are tackling synthetic challenges with four overarching objectives:
- Coordinated synthesis of battery electrode materials, electrode fabrication and assembly of batteries to provide a standard platform, ensuring high quality and reproducibility. This includes the delivery of technique specific materials, e.g., enriched materials for NMR, neutron diffraction.
- Produce designer materials required for specific WP activities, e.g. investigating new electrolyte compositions and chemistries beyond LiPF6, which are less prone to reactivity and can withstand higher voltages; Surface coating of active materials such as silicon particles.
- Utilise our new understanding of degradation processes to mitigate deleterious effects during cycling of batteries. We have set up a feedback loop that feeds the findings from across WPs 1-3 into WP4, informing and guiding our synthetic design. WP4 will thus ensure a continued and constant flow of ideas and information through all WPs.
- Scale-up high quality industrially relevant optimised materials to kg scale quantities.