We start with a quick review of how conventional lithium-ion batteries work.
A lithium-ion battery consists of three main layers: A cathode, or positive electrode, consisting of a lithium-containing mixed-metal oxide material; an anode, or negative electrode, consisting of carbon or a mix of carbon and silicon; a separator, an electrical insulator made of a porous polymer material; and an electrolyte, the medium through which lithium ions move through the battery, typically consisting of a hydrocarbon solvent and dissolved lithium salt.
A battery may be envisioned as the electrochemical equivalent of rolling a ball uphill, which requires work to be put into the system, increasing potential energy in the system during the process, and letting it roll back downhill on its own, to release stored energy and do useful work.
In a fully-discharged cell, the lithium in the cell resides in the cathode, the “downhill” state. When the cell is charged, work is put into the system to drive lithium ions from the cathode to the anode, where they diffuse into the carbon particles that make up the anode. In the fully charged state, the lithium ions sit in the anode, like balls that have been rolled uphill, waiting until they can be freed to roll back downhill again. When the battery is discharged, these lithium ions are allowed to move back from the anode to the cathode, and in the process, energy can be extracted from the system, just like the ball rolling downhill can release its stored energy in the form of useful work.
Anode - Full
Cathode - Spent
Each component needs to be able to hold the other’s load. Therefore, having a higher density cathode without a correspondingly dense anode is useless, since the electron needs to travel back and forth between the anode and cathode.
Starting a fire requires three elements: A fuel, an oxygen source, and a heat source. Because the electrolyte -- a fuel -- is in direct contact with the cathode, which is an oxide, the only other element needed to cause a fire is a heat source. Negative conditions, from internal short-circuits to accidents, can provide that heat source.
The first question to ask when evaluating solid-state cell claims is whether the cells use a lithium-metal anode or a conventional hosted (carbon or carbon-silicon) anode. If they do use a hosted anode, the key performance metrics for these batteries will be similar to conventional lithium-ion batteries and not realize the benefits of the solid-state lithium-metal approach (significantly higher energy density, fast charge, life, and cost).
If the solid-state cell in question does use a lithium-metal anode, the next question to ask is whether it can perform under uncompromised test conditions, including near and below room temperature and high current density (i.e. high rates of power such as 1-hour charge or 15-minute charge). Specifically, what cycle life does the cell deliver at or near room temperature (~ 30°C) with automotive rates of power (>3 mA/cm2, required for one hour charge)? If the cells cannot perform under these conditions, we believe they are not commercially viable. Many of the other solid-state lithium-metal announcements fall into this category.
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