Non-aqueous electrolyte batteries that use a non-aqueous electrolyte such as lithium-ion secondary batteries provide a high energy density at a high voltage and have a reduced size and a reduced weight, and thus are widely used primarily as power sources for information terminals such as personal computers and cellular phones. Common examples of the non-aqueous electrolyte used in the non-aqueous electrolyte batteries include a solution obtained by dissolving a supporting salt such as LiPF6 in an aprotic organic solvent such as an ester compound and an ether compound. Because the aprotic organic solvent is flammable, however, the battery may be ignited when the liquid leaks from the battery or the battery generates an abnormal amount of heat, which is disadvantageous in terms of safety.
The possibility of expanded use of the non-aqueous electrolyte batteries as power sources for large devices such as power sources for power storage and power sources for electric vehicles has recently been studied. Therefore, it has been an important issue to enhance the safety of the non-aqueous electrolyte batteries with a view to increasing the size of the batteries. For example, Japanese Unexamined Patent Application Publication No. 2000-173619 (Patent Document 1) discloses a technology for suppressing ignition or rupture of the non-aqueous electrolyte batteries by covering the surface of a negative electrode with a phosphazene monomer which is a flame retardant material, as a technology for enhancing the safety of the batteries.