The prototyped laminated cell (photo courtesy of NEC)
The prototyped laminated cell (photo courtesy of NEC)
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NEC Corp announced Oct 9, 2012, that it has developed a high-voltage, long-life lithium-ion (Li-ion) rechargeable battery by combining (1) a Ni-Mn-based positive electrode material that is a spinel-type positive electrode material and can have a high voltage and (2) an electrolyte that can withstand a high voltage.

The details of the battery will be announced at PRiME 2012, which is an academic conference that is about electrochemistry and taking place from Oct 7 to 12, 2012, in Hawaii, the US.

The newly developed positive electrode material is Li(Ni0.5Mn1.5)O4, which was made by replacing part of spinel-type lithium manganese oxide (LiMn2O4) with Ni. By using the valency change of Ni, it becomes possible to realize a high voltage.

When the new positive electrode material and graphite are combined, its average operating voltage is 4.5V, which is about 0.7V higher than that of LiMn2O4. As a result, the energy density of a cell can be improved by about 30% from about 150Wh/kg to 200Wh/kg or higher.

However, when the new positive electrode material is combined with an existing polycarbonate (PC)-based electrolyte, its voltage becomes so high, causing oxidative decomposition of the electrolyte. Therefore, NEC newly developed a fluorinated solvent. As a result, it became possible to restrain the oxidative decomposition on the boundary surface between the positive electrode material and the electrolyte and to maintain about 80% of the initial capacity after 500 charge/discharge cycles at a temperature of 20°C.

Furthermore, NEC ensured about 60% of the initial capacity after 500 charge/discharge cycles at a temperature of 45°C. This means a long life equivalent to those of existing Li-ion rechargeable batteries. Also, because gas generation inside the cell can be restrained, the inflation rate of battery after the high-temperature (45°C) cycle test was reduced from 200% (or higher) to about 10%, the company said.

For the development of next-generation Li-ion rechargeable batteries, there is hardly a candidate positive electrode material having a high specific capacity. Therefore, some researchers are developing electrolytes that can withstand high voltages, aiming to improve energy density by increasing operating voltage.