Cathode having improved safety and lithium secondary battery including the same
The present disclosure relates to a cathode including a cathode active material and poly(amic acid). When the lithium secondary battery including the cathode is in an abnormal operation range of 160° C. or higher, condensation reaction of poly(amic acid) occurs to form polyimide. The polyimide surrounds cathode active material particles in the form of a coating film and prevents transport of lithium ions and electrons to interrupt current flow, and inhibits abnormal heat emission and thermal runaway or combustion caused by additional reactions.
1. A cathode comprising a cathode active material and poly(amic acid) represented by the following Chemical Formula 1a:
wherein R independently represents a C1-C12 alkenyl or C6-C12 aromatic ring, and a carbon atom or hydrogen atom of the alkenyl or aromatic ring is non-substituted or substituted with an oxygen, sulfur or halogen atom depending on atomic valences; and
A independently represents a bond, C1-C12 alkenyl or C6-C12 aromatic ring, and a carbon atom or hydrogen atom of the alkenyl or aromatic ring is non-substituted or is substituted with an oxygen, sulfur or halogen atom depending on atomic valences,
wherein the poly(amic acid) is included in an amount of 1 part to 5 parts by weight based on 100 parts by weight of the cathode active material.
2. The cathode according to claim 1 , wherein the poly(amic acid) is converted into polyimide at a temperature of 160° C. or higher.
3. The cathode according to claim 1 , wherein the poly(amic acid) is represented by the following Chemical Formula 2a:
4. The cathode according to claim 1 , wherein, as a binder polymer, only the poly(amic acid) represented by the above Chemical Formula 1a is comprised.
5. The cathode according to claim 1 , wherein the poly(amic acid) has a molecular weight of 5,000-200,000.
6. The cathode according to claim 1 , which further comprises microcapsules comprising: a shell having a glass transition temperature or melting point at 160° C.; and pyridine and diacetic anhydride present inside of the shell.
7. A lithium secondary battery comprising the cathode of claim 1 .