Flame Retardant Additive in Electrolyte-Prevent Lithium Battery Burning
In order to reduce the flammability of the electrolyte, adding a small amount of flame retardant additive to the electrolyte of the lithium ion battery can effectively improve the thermal stability of the electrolyte, thereby improving the safety of the lithium ion battery.The use of flame retardant additives is simple and effective, and is a research hotspot of liquid organic electrolytes for lithium ion batteries.
The organic solvent in the electrolyte causes combustion due to the chain reaction itself,The flame retardant is based on the principle of catalytic removal of free radicals (such as H. or .OH) with high reactivity and combustion reaction, inhibiting the occurrence of these chain reactions, and effectively improving the safety of lithium ion batteries. These free radicals maintain a gas phase combustion reaction through a cascade of propagation mechanisms.
Flame retardants terminate this combustion reaction by two possible mechanisms：
1 chemical reaction
the flame retardant decomposes at high temperature to form free radicals (such as PO2. or HPO2.), and terminates the chain-propagation reaction by removing H. or .OH radicals;
2 Physical process
the flame retardant forms a thermal insulation layer between the condensed state and the gas phase, preventing the combustion reaction from proceeding.At present, the flame retardant mechanism generally accepted is the hydrogen radical trapping mechanism.
Common lithium-ion battery electrolyte flame retardant additives mainly include (s)phosphoric acid ester flame retardants and phosphorus-nitrogen (P-N) composite flame retardants.They are effective in inactivating free radicals from the decomposition of organic carbonates and destroying the combustion caused by chain-propagating reactions.The phosphate ester flame retardants mainly include some alkyl phosphates and phenyl phosphate compounds.These compounds are liquid at room temperature and have a certain mutual solubility with organic media. They are important flame retardant additives for lithium ion battery electrolytes.
Trimethyl phosphate (TMP) and triethyl phosphate (TEP) are among the first flame retardants to be studied.However, TMP and TEP are not stable on the graphite negative electrode at a low reduction potential.Due to the higher phosphorus content in TMP, TMP has a better flame retardant effect than TEP.However, its compatibility with graphite anodes is even worse, which means that a balance between flame retardancy and reduction stability is sought.The flammability of the electrolyte is reduced due to the addition of the flame retardant, but this is at the expense of other properties of the battery, such as the ionic conductivity of the electrolyte and the reversibility of the battery.
Ways to improve the reduction stability of phosphate esters are:
1 Increase the carbon atom on the alkyl group;
2 Replacing a part of the alkyl group with a phenyl group;
3 Construct a cyclic phosphate structure.
Studies have shown that phosphite compounds also have very good flame retardant effects.
The advantages of phosphite flame retardants compared to phosphate flame retardants are:、
1 Conducive to the formation of SEI film;
2 It can inactivate PF5, prevent it from damaging the SEI membrane, and keep the electrode stable.
The composite flame retardant utilizes the synergistic effect between different flame retardant elements to improve the flame retardant effect and reduce the amount of the flame retardant.
The composite flame retardant commonly used in lithium ion battery electrolytes is mainly P-N flame retardant.A cyclophosphazene compound is an ideal flame retardant.The advantages of this compound include:
1 Due to the existence of the annular structure, the phosphorus content in the flame retardant is higher, and the flame retarding effect is improved;
2 Excellent stability at both the positive electrode and the graphite negative electrode at a lower potential.