Lithium-ion batteries are essential components in a number of established and emerging applications including: consumer electronics, electric vehicles and grid scale energy storage. However, despite their now widespread use, their performance, lifetime and cost still needs to be improved. The ESE group works at a range of multi-disciplinary length scales to solve these problems with activities including: development of new materials, characterisation of these materials, modelling of their performance, thermal management of cells, development of control systems, analysis of battery pack designs, diagnostic techniques and techno-economic analysis.

Projects

Modelling/experimental validation of mechanical effects in lithium-ion batteries

Faraday Institution / Innovate UK logo
Lead researchers: Dr Weilong Ai and Dr Jingy Chen
PI’s: Dr Billy Wu and Dr Gregory Offer
 
Funding: 
  • EPSRC Faraday Institution Multi-Scale Modelling project (EP/S003053/1, grant number FIRG003)
  • Innovate UK Battery Advances for Future Transport Applications (BAFTA) Project, collaborative R&D with Aston Martin Lagonda and Dukosi Limited.

Electrode materials experience large volume change during lithium (de)intercalation, and the resulting stress can cause cracking in electrode particles. Those cracks release new surface area to allow side reactions including solid electrolyte interphase growth and lithium plating, which accelerate the capacity degradation of lithium ion batteries. This project is to investigate the root causes of particle cracking and to develop models to improve battery performance by alleviating particle cracking issues. 

fig 1 c-rates
Figure 1. Nonuniform stress distribution across electrodes at different C-rates of discharging

The latest findings include a developed stress model for batteries showing large stresses close to the separator for high C-rates (Figure 1) and nonlinear diffusivity of electrode materials causing severe particle cracking in a narrow range of state of charge. In line with the non-linear diffusivity, cell aging experiments are on-going to investigate the influence of SOC range, temperature and C-rate on the rate of degradation. Electron imaging on the electrode particles serves as direct evidence of the extent of cracking (Figure 2) in post-mortem analysis.

Non-invasive in-situ diagnostic techniques, incremental capacity analysis (ICA) and differential thermal voltammetry (DTV) are applied to quantify extent of cell degradation, which provides insights in real-time battery management for maximising both performance and longevity. Long-term test data are collected from in-house facility (Figure 3).

 
Collaborators:
  • ESE group - Dyson School of Design Engineering: Mr Max Naylor Marlow
  • Department of Civil and Environmental Engineering: Dr Emilio Martinez-Paneda
Publications:
  1. Ai W, Kraft L, Sturm J, Jossen A, Wu B, 2019, Electrochemical Thermal-Mechanical Modelling of Stress Inhomogeneity in Lithium-Ion Pouch Cells, Journal of The Electrochemical Society, Vol: 167, Pages: 013512 - 013524

Conference presentations:
  1. Ai W, Chen J, O’Connor TS, Marlow MN, Wu B, An electrochemo-mechanical coupled model for lithium ion batteries, Oxford Battery Modelling Symposium (OBMS 2020), March 16-17, 2020, Oxford, UK.

Battery pack modelling

Faraday Institution logo
Lead researchers: Mr Max P Naylor Marlow and 
Dr Weilong Ai
PI’s: Dr Billy Wu
 
Funding: 
  • EPSRC Faraday Institution Multi-Scale Modelling project (EP/S003053/1, grant number FIRG003)
  • Imperial College London Doctoral Training Partnership (EP/N509486/1 project number 1854850) 

The performance of lithium-ion battery packs are often extrapolated from single cell performance however uneven currents in parallel strings due to cell-to-cell variations, thermal gradients and/or cell interconnects can reduce the overall performance of a large scale lithium-ion battery pack. This project is to investigate the performance implications caused by those factors by experiments and simulations:

fig 1 test bench
Figure 1. The photo of the test bench for 6x 2P1S parallel strings configuration

Experimental: Experimental work includes parameterisation and validation of models at the pack level, using our in-house custom pack test bench (Figure 1) to evaluate the behaviour of parallel cell strings under applied thermal conditions

fig 2 pack performance
Figure 2. Battery pack performance: (a) current distribution across six battery cells and (b) output energy and degradation for battery packs with different thermal gradients

Theoretical: Thermally coupled single particle models are developed to analyse the performance of battery packs, with considering the growth of solid electrolyte interphase layers. The output energy and capacity degradation of battery packs are studied under different operating conditions (Figure 2).

This work will provide insights for the designs of higher performance and longer lasting battery packs.

Collaborators:
  • UCL - Dept of Chemical Engineering: Dr Thomas Tranter
Publications:
  1. Liu X, Ai W, Marlow MN, Patel Y, Wu B, 2019, The effect of cell-to-cell variations and thermal gradients on the performance and degradation of lithium-ion battery packs, Applied Energy, Vol: 248, Pages: 489-499.

Modelling of Electrochemical–Thermal-Mechanical Degradation in Lithium-Ion Batteries

Faraday Institution / Innovate UK logo
Lead researchers: Dr Ganesh Madabattula, Dr Simon O’Kane, Dr Weilong AiDr Abir Ghosh and Mr Jason Jiang
PI’s: Dr Gregory Offer, Dr Monica MarinescuDr Billy Wu and Dr Huizhi Wang
 
Funding: 
  • EPSRC Faraday Institution, Multi-Scale Modelling project (EP/S003053/1, grant number FIRG003)
  • Innovate UK, WIZer Batteries project

fig 1 diff voltage signals
Figure 1. Differential voltage signals predicted by the reversible Li plating model (O’Kane et al. 2020). The DV signal becomes significantly sharper if plating is enabled in the model.

Having already powered a revolution in portable electronics, lithium-ion batteries are expanding into bigger applications such as electric vehicles, storing surplus electricity from renewable sources and backup batteries for when mains electricity fails. However, as the energy and power of batteries increase, batteries make up a larger fraction of the product’s final cost and lifetime is, therefore, more of a concern to manufacturers and consumers. Battery degradation has many different causes including unwanted electrochemical side reactions, self-heating and mechanical wear and tear. Being able to detect and prevent degradation requires a greater physical understanding of each mechanism than we currently have. One way of gaining this understanding is physics-informed computer modelling (Figure 1).

Our group works with and contributes to the open-source modelling platform PyBaMM (Python Battery Mathematical Modelling), which was created by the University of Oxford Mathematical Institute.

Our experimental colleague Dr Niall Kirkaldy has assembled custom rigs that allow many different degradation mechanisms to be performed on different cells from a batch.

Collaborators:
Publications:
  1. O’Kane SEJ, Campbell ID, Marzook WJ, Offer GJ and Marinescu M, 2020, Physical Origin of the Differential Voltage Minimum Associated with Lithium Plating in Li-Ion batteries, Journal of the Electrochemical Society, Vol: 167, Pages: 090540. 

Modelling Li-ion battery fast charging

Shell logo
Lead researcher: Miss Anna Tomaszewska
PI’s: Dr Billy Wu and Dr Gregory Offer
 
Funding: 
  • Shell UK

Range anxiety is often quoted as a key barrier to the widespread adoption of electric vehicles. To compete with internal combustion engines, batteries must be capable of recharging to 80% state of charge in about 10 minutes. However, fast charging is known to result in accelerated capacity and power fade particularly in high energy cells, where the transport of lithium ions is hindered by thicker electrodes. When the rate of current supply outpaces the rate of ion diffusion, lithium may begin depositing on the anode instead of intercalating into it, leading to the loss of lithium inventory, local electrode deactivation, and internal short circuits in extreme cases.

The project focuses on model-based approaches to optimize the fast charging performance of Li-ion batteries. The first outcome was a comprehensive review of the existing fast charging strategies from the material to cell and pack level. Although many promising material improvements were identified, engineering solutions such as current profile and thermal management optimization were found to be achievable over much shorter timescales and at a lower cost. The project aims to address two of the key knowledge gaps identified in the review:

fig 1 key factors Li-ion batteries
Figure 1. Key factors affecting Li-ion battery fast charging at different length scales
  • Out of the many fast charging protocols proposed in the literature, very few have been evaluated at temperatures other than room temperature or for more than one cell type.

  • The understanding of fast charging performance under realistic thermal conditions is very limited. Most studies have been conducted under isothermal conditions or in a climate chamber, while individual cells in battery packs can experience significant transient and spatial thermal gradients.
fig 2 UVP results
Figure 2. Electrolyte concentration and current density profiles during fast charging with CC-CV and UVP at 25⁰C and -10⁰C. At subzero temperatures, UVP results in more severe electrolyte depletion which leads to a more heterogenous current distribution and increased rate of lithium plating

Initial work on physics-based modelling of fast charging protocols has shown that charging profiles can perform differently under different environmental conditions or for different cells. For example, a charging protocol known as the Universal Voltage Protocol (UVP), previously demonstrated to significantly reduce degradation when used at room temperature, has been shown to be potentially more damaging than standard constant current – constant voltage (CC-CV) charging at subzero temperatures in high energy cells due to electrolyte depletion effects.

Recently, the model has been extended to 2D and 3D to investigate the effects of thermal gradients that typically arise in battery packs on the lithium plating behaviour, including both the plating distribution and severity compared to a thermally uniform cell. Experimental validation is ongoing. The findings of the project will hopefully inform the selection of appropriate fast charging strategies based on the ambient conditions, thermal management layout, and cell type.

Publications:
  1. Tomaszewska A, Chu Z, Feng X, O’Kane S, Liu X, Chen J, Ji C, Endler E, Li R, Liu L, Li Y, Zheng S, Vetterlein S, Gao M, Du J, Parkes M, Ouyang M, Marinescu M, Offer G, Wu B, 2019, Lithium-ion battery fast charging: A review, eTransportation, Vol: 1, Page: 100011.

New electrolytes for lithium batteries

EPSRC logo
Lead researcher: Dr Yan Zhao
PI: Dr Huizhi Wang
 
Funding: 
  • EPSRC UK grant, Smart Microfluidics Towards Low-Cost High-Performance Li-Ion Batteries (EP/S000933/1)

Metallic Li anode can react with carbonate electrolytes, which can lead to continuous side reactions between Li and electrolytes during repeated Li plating/stripping, and eventually cause low Li Coulombic efficiency and poor cycle life. Ether-based electrolytes are regarded as an excellent candidate because of their high reductive stability with Li. However, the oxidation instability of ethers beyond 4.0V have limited their application in practical high-voltage lithium metal batteries. This research is to develop an ideal electrolyte system for high-voltage metal Li batteries by investigating novel Li salts or electrolyte additives.

fig 1 SEI effects
Figure 1. Effect of different SEIs or CEIs on Li anodes or high-voltage cathodes. (a) Conventional organic-rich SEIs cannot (b) while LiF-rich SEIs can suppress the Li dendrite growth. (c) Conventional organic-rich CEIs cannot (d) while LiF-rich CEIs can suppress the interactions between electrolytes and cathodes.

The latest common sense about electrolytes is to develop high-concentration electrolytes, which can effectively enhance their oxidation stabilities and the cycle life of high-voltage electrodes. However, the high concentration electrolytes are facing several challenges such as high viscosity and subsequent low ionic conductivity as well as high cost. The latest finding is that some fluorinated solvents or additives can contribute to the formation of stable LiF-rich SEI on metal Li surface or CEI on high-voltage cathode surface (Figure 1), which can improve the stable cycling of high-voltage Li metal batteries.

X-ray photoelectron spectroscopy (XPS) is conducted to analyse the surface composition of SEIs or CEIs. Scanning electron microscope (SEM) and transmission electron microscope (TEM) are used to characterize the morphology and structure of the samples. The electrochemical properties are completed by the electrochemical workstation and the cycling data of the cells are collected on Land battery testers.

Collaborators:
  • ESE group - Department of Mechanical Engineering: Mr Yang Jiang
Publications:
  1. To be updated.

Recent publications 2018 - to date


Bravo Diaz L, He X, Hu Z, Restuccia F, Marinescu M, Varela Barreras J, Patel Y, Offer G, R, 2020, Review—Meta-Review of Fire Safety of Lithium-Ion Batteries: Industry Challenges and Research Contributions, Journal of the Electrochemical Society, Vol: 167, Pages: 090559

Ai W, Kraft L, Sturm J, Jossen A, Wu B, 2020, Electrochemical Thermal-Mechanical Modelling of Stress Inhomogeneity in Lithium-Ion Pouch Cells, Journal of the Electrochemical Society, Vol: 167, Pages: 13512. 

Zhao Y, Bravo Diaz Laura, Patel Y, Zhang T, Offer GJ, 2019, How to Cool Lithium Ion Batteries: Optimising Cell Design using a Thermally Coupled Model, Journal of the Electrochemical Society, Vol: 166, Pages: A2849-A2859. 

Hales A, Bravo Diaz L, Marzook MW, Zhao Y, Patel Y, Offer G, 2019, The Surface Cell Cooling Coefficient: A Standard to Define Heat Rejection from Lithium-Ion Batteries, Journal of the Electrochemical Society, Vol: 166, Pages: A2383-A2395. 

Zhao Y, Spingler FB, Patel Y, Offer GJ, Jossen A, 2019, Localized Swelling Inhomogeneity Detection in Lithium Ion Cells Using Multi-Dimensional Laser Scanning, Journal of the Electrochemical Society, Vol: 166, Pages: A27-A34. 

Campbell ID, Marzook M, Marinescu M, Offer GJ, 2019, How Observable Is Lithium Plating? Differential Voltage Analysis to Identify and Quantify Lithium Plating Following Fast Charging of Cold Lithium-Ion Batteries, Journal of the Electrochemical Society, VOl: 166, Pages: A725-A739. 

Song W, Liu X, Wu B, Brandon N, Shearing PR, Brett DJL, 2019, Sn@C evolution from yolk-shell to core-shell in carbon nanofibers with suppressed degradation of lithium storage. In Energy Storage Materials 18, pp. 229–237.

The Cell Cooling Coefficient for Li-ion Batteries

The Cell Cooling Coefficient

Check out our short animation explaining this battery design tool

The cell cooling coefficient is a metric which can aid the design of the next generation of battery packs and highlight the importance of good thermal management across the battery industry.

Selected open access publications: The surface cell cooling coefficientThe tab cell cooling coefficient and How to cool Li-ion batteries