Silicon is also not intrinsically elastic enough to cope with the strain of lithiation when it is repeatedly charged, leading to cracking, pulverization and rapid physical degradation of the anode’s composite microstructure. This contributes significantly to capacity fade, along with degradation events that occur on the counter electrode—the cathode.

Numerous approaches have attempted to overcome these issues, including the use of nano-sized / structured silicon particles with micron-sized graphene. This, however, has not proved satisfactory. Using nano-sized silicon particles significantly increases the amount of reactive surface available, which leads to much more lithium being deposited on the silicon during the first charge cycle forming a solid-electrolyte interphase barrier between the silicon and the electrolyte and thus greatly reducing the lithium inventory and thus the battery’s useful lifetime.

This layer also continues to grow on silicon and so the lithium loss becomes continuous. Other methods of incorporating other materials such as graphene at different sizes have been deemed impractical to then progress to large–scale manufacture.

The new research, led by Dr. Melanie Loveridge in WMG at the University of Warwick, has discovered, and tested, a new anode mixture of silicon and a form of chemically modified graphene which could resolve these issues and create viable silicon anode lithium-ion batteries. Such an approach could be practically manufactured on an industrial scale and without the need to resort to nano sizing of silicon and its associated problems.

Schematic of FLG preventing Si electrochemically “fusing” together. The FLG flakes preserve the degree of separation between the silicon particles with each battery charge cycle. Source: WMG. Click to enlarge.

Graphene is a single, one-atom-thick layer of the mineral graphite (an allotrope of carbon). However, it also possible to separate and manipulate a few connected layers of graphene giving a material researchers refer to as few-layer graphene (FLG).

Previous research has tested the use of FLG with nano-sized silicon but this new study has found that FLG can also improve the performance of larger micron-sized silicon particles when used in an anode. This mixture could significantly extend the life of lithium-ion batteries and also offer increased power capability.

The researchers created anodes that were a mixture of 60% micro silicon particles, 16% FLG, 14% Sodium/Polyacrylic acid, and 10% carbon additives, and then examined the performance (and the changes in structure of the material) over 100 charge-discharge cycles.

A cross section of the silicon and FLG together in an anode. Source: WMG. Click to enlarge.

The flakes of FLG were mixed throughout the anode and acted like a set of strong, but relatively elastic, girders. These flakes of FLG increased the resilience and tensile properties of the material greatly reducing the damage caused by the physical expansion of the silicon during lithiation. The graphene enhances the long range electrical conductivity of the anode and maintains a low resistance in a structurally stable composite.

More importantly, these FLG flakes can also prove very effective at preserving the degree of separation between the silicon particles.

—Dr. Melanie Loveridge

The WMG research team have already begun further work on this advance which will include further study and research as part of a two-year project led by Varta Micro-innovations. WMG at the University of Warwick is a partner along with Cambridge University, CIC, Lithops and IIT (Italian Institute of Technology). The main goal of that project is to advance in pre-industrial production of silicon/graphene composites and their subsequent processing into lithium-ion batteries for high-energy and high-power applications.

As part of that project WMG at Warwick will be optimizing the electrode research, scale up and pouch cell manufacture of the optimized Li-ion batteries.

Resources

• Qianye Huang, Melanie J. Loveridge, Ronny Genieser, Michael J. Lain & Rohit Bhagat (2018) “Electrochemical Evaluation and Phase-related Impedance Studies on Silicon–Few Layer Graphene (FLG) Composite Electrode Systems” Scientific Reports 8, Article number: 1386 doi: 10.1038/s41598-018-19929-3