Advance in understanding of all-solid-state batteries

July 31, 2019

HARWELL, UK (July 29, 2019)--All-solid-state batteries, a battery design composed of all solid components, have gained attention as the next major advance beyond lithium ion batteries because of their potential to store more energy while being safer to operate. When capable of being produced in commercial quantities, solid-state batteries would revolutionise electric vehicles (EVs) effectively increasing driveable range or significantly decreasing volume and weight.

Yet solid-state batteries can fail after cycling (repeated charging and discharging) at practical currents, which has been one of the barriers preventing their mass commercialisation.

In a new paper published by Nature Materials entitled "Critical Stripping Current Leads to Dendrite Formation on Plating in Lithium Anode Solid Electrolyte Cells," Faraday Institution researchers at the University of Oxford have taken a step forward in understanding the mechanisms by which solid-state batteries fail--a necessary prerequisite to avoiding such failures.

Dendrites are branching networks of lithium which grow through the solid, ceramic, electrolyte during charging of a battery, causing a short circuit.

"This research adds to our fundamental knowledge of why solid-state batteries behave the way they do. We believe our improved understanding will help to inform approaches to avoid some of the problems at the lithium metal anode in solid electrolyte cells," commented Professor Peter G. Bruce of the University of Oxford's Departments of Materials and Chemistry and Principal Investigator of the Faraday Institution's SOLBAT project, whose team led the research.

Void creation at the anode of solid-state cells has long been recognised, but their role in dendrite formation has not been understood. The study uses a combination of cutting edge electrochemical and imaging techniques to form a fundamental understanding of void formation as a function of cycling and its role in dendrites and cell failure.

Significantly, parameters in the failure model correlate to key physical properties that could be used as "levers" to suppress void formation and cell failure.

"It is key to break down the scientific barriers that prevent the progression to market of technologies that will enable our vision of the future of mobility. The study by Oxford researchers is one early example of a scientific advance that the Faraday Institution was set up to drive," said Tony Harper, Director of the ISCF Faraday Battery Challenge at UK Research & Innovation.

The research: importance of critical current density on stripping

One well-recognised challenge facing scientists studying solid-state batteries is to prevent dendrite growth as batteries are cycled between a charged and uncharged state (as they would repeatedly have to do if they to be used to power EVs).

Another significant problem is void formation between the solid electrolyte and lithium anode (negatively charged electrode) during stripping (discharge of a battery), which leads to a reduced area of contact between those two parts of the battery cell.

It is difficult to separate lithium plating from stripping using an experiment with a battery cell containing the usual two electrodes. In this study researchers used three-electrode cells to study separately the processes of plating and stripping lithium metal at the lithium metal / ceramic interface on battery cycling. Argyrodite, Li6PS5Cl, was chosen as the solid electrolyte. Such sulphides have higher conductivity than oxides and are being pursued as the electrolyte of choice by several companies attempting to commercialise solid-state batteries. Argyrodite has the advantage of being less brittle than other highly conducting sulphides.

The researchers found that if dendrite formation is to be avoided in all-solid-state battery cells, it is vital to cycle the cells below the critical current density at which voids begin to form at the lithium metal / solid electrolyte interface during lithium stripping (CCS). This is the case even when the current density is below the threshold for dendrite formation on plating. When the current density is greater than CCS, voids accumulate on cycling, the lithium / solid electrolyte area of contact decreases correspondingly and as a result the local current density increases until it reaches a value where dendrites form on plating, leading to a short-circuit and cell failure. It may take multiple cycles, but the research demonstrates that cell failure is inevitable if the overall current density is greater than CCS. These results show that it is not just the current density for dendrite formation that is important in achieving cycling of all-solid-state cells at practical current densities; stripping currents are also important.

The researchers also conclude the lithium metal creep is the primary mechanism of transporting lithium metal at the interface.

The team working on this discovery included a mix of theoreticians and experimentalists, in the type of multi-disciplinary research environment that the Faraday Institution fosters.

*The prizes of developing a commercial all solid-state battery for electric vehicles*

Small, non-rechargeable, solid-state batteries are growing in commercial use, for example, in medical implants such as heart monitoring. However, there remain considerable challenges associated with both the fabrication of solid-state batteries at the scale required for use in EVs, and to ensure that such devices operate safely and to acceptable performance levels over the lifetime of the EV.

Current lithium ion batteries used in EVs contain a flammable organic liquid electrolyte, through which charge-carrying lithium ions pass during the charging and discharging of the battery. This liquid presents an inherent (albeit well-managed) safety concern. The replacement of the liquid electrolyte with a solid would remove this fire risk.

Worldwide, significant scientific effort is being expended to develop new battery chemistries that would achieve battery performance (power density and energy density) that would give an EV driving experience aligned with expectations from driving cars with internal combustion engines. The elimination of the need for a liquid electrolyte would be a prerequisite for developing batteries with a lithium metal anode, which could unlock significant performance improvements.
*The Faraday Institution's research projects*

Along with the SOLBAT project that is addressing the fundamental scientific barriers facing the commercial realisation of solid-state batteries, the Faraday Institution launched three other large, multi-disciplinary research projects in 2018 to: enhance battery life; develop multi-scale models of battery, and; define recycling and reuse strategies. They will be joined by additional projects later in 2019 to research: next-generation lithium ion cathode materials, electrode manufacturing, sodium ion batteries and alternative cell chemistries beyond lithium ion.

Notes to Editors

Powering Britain's battery revolution, the Faraday Institution is the UK's independent institute for electrochemical energy storage science and technology, supporting research, training, and analysis. Bringing together expertise from universities and industry, the Faraday Institution endeavours to make the UK the go-to place for the research and development of the manufacture and production of new electrical storage technologies for both the automotive and wider relevant sectors.

The first phase of the Faraday Institution is funded by the Engineering and Physical Sciences Research Council (EPSRC) as part of UK Research and Innovation through the government's Industrial Strategy Challenge Fund (ISCF). Headquartered at the Harwell Science and Innovation Campus, the Faraday Institution is a registered charity with an independent board of trustees.

The University of Oxford is committed to energy research, including research on energy storage science and technology. It is making a significant investment in this area with new academic appointments and new state of the art laboratories. Peter Bruce, Wolfson chair in Materials, is leading this effort along with colleagues Mauro Pasta, Charles Monroe and David Howey. His own research group focuses on the fundamental science of electrochemical energy storage, especially lithium and sodium batteries.

The ISCF Faraday Battery Challenge is to develop and manufacture batteries for the electrification of vehicles--£274 million over four years --to help UK businesses seize the opportunities presented by the move to a low carbon economy. The challenge will be split into three elements: research, innovation, and scale-up. It is among the first of six investment areas announced by the government to be funded through the Industrial Strategy Challenge Fund.

The Engineering and Physical Sciences Research Council (EPSRC) is part of UK Research and Innovation, a non-departmental public body funded by a grant-in-aid from the UK government.

EPSRC is the main funding body for engineering and physical sciences research in the UK. By investing in research and postgraduate training, we are building the knowledge and skills base needed to address the scientific and technological challenges facing the nation.

Our portfolio covers a vast range of fields from healthcare technologies to structural engineering, manufacturing to mathematics, advanced materials to chemistry. The research we fund has impact across all sectors. It provides a platform for future UK prosperity by contributing to a healthy, connected, resilient, productive nation.

Industrial Strategy Challenge Fund: The Industrial Strategy Challenge Fund aims to bring together the UK's world leading research with business to meet the major industrial and societal challenges of our time. The fund was created to provide funding and support to UK businesses and researchers, part of the government's £4.7 billion increase in research and development over the next 4 years. It was designed to ensure that research and innovation takes centre stage in the Government's modern Industrial Strategy. It is run by UK Research and Innovation.

UK Research and Innovation is a new body which works in partnership with universities, research organisations, businesses, charities, and government to create the best possible environment for research and innovation to flourish. We aim to maximise the contribution of each of our component parts, working individually and collectively. We work with our many partners to benefit everyone through knowledge, talent and ideas.

Operating across the whole of the UK with a combined budget of more than £6 billion, UK Research and Innovation brings together the seven Research Councils, Innovate UK and a new organisation, Research England.

The Faraday Institution

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