Recently, NIO and SAIC have successively proposed models with a range of 1000 kilometers in the future, causing a huge uproar in the power battery industry. Taking advantage of this trend, we will explore how lithium batteries can be researched and upgraded on negative electrode materials to improve the overall energy density of the battery and achieve higher range.

The current negative electrode materials for lithium batteries are mainly based on graphite, which serves as a carrier for lithium ions and electrons during battery charging. The layered structure of graphite can be imagined as a bookshelf in a library. During charging, lithium ions enter from the positive electrode to the negative electrode, and then the lithium ions are inserted into the graphite bookshelf in a ratio of 6 carbon atoms fixed to 1 lithium ion (LiC6). This ratio determines that the maximum energy storage density of graphite negative electrode materials is 372 milliampere hours per gram.

One way to improve the energy density of negative electrode materials is to find bookshelf materials that can accommodate more dense lithium ions. Researchers have found that silicon materials can store lithium ions in a ratio of 4 silicon atoms to 15 lithium ions (Li15Si4), thereby increasing the maximum energy storage density of negative electrode materials to 3579 milliampere hours per gram, which is 9.6 times higher than the maximum energy storage density of graphite negative electrodes.

While materials bring such a huge increase in energy density, they also bring a huge problem: the increase in the amount of lithium ions stored can cause a sharp expansion of the volume of silicon materials, just like the bookshelf will be stretched out after inserting too many books. Graphite negative electrode materials can expand up to 40% in volume after absorbing lithium ions, while silicon materials can expand up to 360%. The huge volume expansion has brought some practical difficulties, and the following figure shows the three main problems:


1. Pulverization of silicon particles: After absorbing lithium ions, the volume of silicon particles expands, and after releasing lithium ions, the volume shrinks. Due to the rigidity of silicon particles, repeated expansion and contraction can lead to particle fragmentation. The shattered part no longer maintains current communication with the electrode, becoming useless material and reducing the energy storage capacity of the battery.

2. Unstable solid electrolyte interface facial mask (Unstable SEI): When the graphite particles are charged for the first time, a layer of solid electrolyte interface facial mask will be formed on the surface of the graphite particles. This layer of film plays the role of protecting the internal graphite materials and the external electrolyte, to prevent them from continuing to react with each other and consuming useful ingredients. The volume of graphite particles changes little during charging and discharging, and the facial mask can remain unbroken depending on its own toughness, so it will not change since the first charging. After being replaced with silicon particles, due to the large volume change of silicon particles, the facial mask will be damaged each time the discharge shrinks in the initial charge discharge cycle, and a new layer of film will continue to form on the broken film in the next charge. The repeated increase in thickness of the membrane seriously hinders the entry of lithium ions and electrons into the silicon particles inside the membrane, while also consuming a large amount of silicon material inside the membrane and electrolyte outside the membrane, rapidly reducing the battery's energy storage capacity and ultimately leading to battery failure.

3. Electrode failure: In order for the negative electrode material to function, it is necessary to maintain current flow with the copper foil of the electrode, so that electrons can be transferred to the negative electrode material through the copper foil. At the beginning, silicon particles can maintain conductivity with copper foil due to their close stacking and contact with each other. After repeated charging and discharging, due to the expansion and contraction of the silicon particles themselves, the distance between the particles will gradually widen to form gaps. The particles that were originally tightly attached to the copper foil will also gradually fall off, causing the negative electrode to lose conductivity inside and resulting in battery failure.

Due to the above issues, pure silicon negative electrode materials are currently not feasible, but people have been trying to replace some graphite materials with silicon materials to make silicon carbon composite negative electrodes. The current substitution ratio ranges from 5% to 20%, which also increases the energy storage density of negative electrodes by 50% to 150%. However, even with a small amount of replacement graphite materials, people still need to address the impact of volume expansion of silicon materials on the negative electrode. The most commonly used solution currently is to use nanoscale porous silicon materials to reserve enough space inside the silicon particles, so that when the volume expands, it occupies the reserved internal space without causing significant changes in the external volume, thereby maintaining the stability of the negative electrode.

We will explain in future articles how to design porous silicon particles and how to maintain good integration with graphite materials, electrolytes, and adhesives in the negative electrode. We will also introduce research institutions and companies engaged in related research and development work.

Reference:
1. Silicon Carbon composite nodes from industrialbattery grade silicon. Scientific Reports 9, Article number: 14814 (2019)
2. Engineering of carbon and other protective coating layers for stabilizing silicon anode materials. Carbon Energy 1, 219-245 (2019)
Author Introduction:
Dr. Xie Wei, Bachelor and Master of Materials Science from Tsinghua University, and Ph.D. in Chemical Engineering from the University of Texas (Austin) in the United States. Mainly engaged in the development of energy storage batteries, has held important positions in multinational corporations and startups, led multiple research and development projects funded by the US Department of Energy, and won the 2013 US Annual 100 Best Research and Development Technology Award. Published 17 papers in top journals in materials science and energy storage, served as a reviewer for 5 international journals, and has applied for 17 international invention patents.