Regardless of the battery type, safety depends on the following factors:
How high is the risk of Internal Short Circuit (ISC)?
Is there a certain amount of combustibles present or generated?
Is the spontaneous combustion duration long enough?
The current mainstream positioning in the industry is:
Sulfide solid electrolytes are used for All-Solid-State Batteries,
and Oxide solid electrolytes are used for Semi-Solid-State Batteries.
All-Solid-State Batteries:
Judging by current technological progress, Lithium Dendrites have not been eliminated, especially when Lithium Metal is used as the anode. This indicates that the risk of internal short circuits remains high. The existence of an internal short circuit implies high internal temperatures. Sulfides may decompose into gas (toxic) at 200°C, and this gas is also flammable. The proportion of sulfides inside an all-solid-state battery is significant; once ignited, it will burn for a period of time.
Semi-Solid-State Batteries:
The proportion of liquid electrolyte is 5~10%, and a separator is used. Lithium dendrites still exist, and the risk of internal short circuits also exists. An internal short circuit implies the possibility of high internal temperatures. The solvent in the electrolyte evaporates to become flammable gas, which will also sustain combustion.
We often see videos online showing that needle penetration test results for semi-solid-state batteries are similar to Lithium Iron Phosphate (LFP) batteries—they do not spontaneously combust!
This is because the severity of the internal short circuit is relatively mild. However, if multiple batteries are connected in Series, when one cell develops an internal short circuit during charging or discharging, the short-circuit current of that cell will increase significantly, especially during high-current charging/discharging. As the severity of the internal short circuit increases, the internal short-circuit current increases noticeably, and the internal temperature rise becomes faster and higher, thereby naturally increasing the risk of spontaneous combustion.
This is also why a single LFP battery is safe during needle penetration, but there is still a possibility of spontaneous combustion after being assembled into a PACK.
In summary, for all batteries where “Internal Short Circuits are possible,”