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| Input and output power characteristics of 2nd and 3rd generations. Source: Hitachi Review. Click to enlarge. |
GM’s eAssist light electrification system—currently offered as the base powertrain in the full-sized 2012 Buick LaCrosse and offered on the 2012 Buick Regal—uses a 115V, 0.5 kWh lithium-ion battery system built with Hitachi third-generation power-optimized cells. (Earlier post.)
In a paper in the current issue of Hitachi Review, Hitachi engineers describe the characteristics of these cells—targeted at hybrid electric vehicle applications—as well as their current efforts on the coming fourth-generation of power-optimized cells for hybrids (earlier post) and efforts on a prismatic cell for plug-in hybrid electric vehicles.
The third-generation cylindrical cells feature a manganese-based cathode and amorphous carbon anode, and offer a high output power density of 3,000 W/kg together with small size and light weight. In developing the third-generation, Hitachi engineers optimized the proportion of lithium and other metals in the cathode and controlled the microstructure to reduce the charge transfer resistance of the active material surface.
The battery’s structure was also designed to minimize the length of the cathode and anode leads. Improvements to the welding methods helped to reduce the electrical resistance.
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| Calendar life comparison. Click to enlarge. |
Improvements in the anode structure and new electrolyte additives resulted in improved calendar life over the prior generation of cells; the improvement in lifecycle of batteries kept at high temperature allowed the battery size to be reduced.
Hitachi also developed packs to GM’s requirements. Each pack consists of two 16-cell modules comprising two blocks of eight cells with the battery monitoring module located on top of the battery modules. The pack is air-cooled; cell layout, cooling flow, and other parameters were optimized to reduce cell temperature rise, minimize the variation in temperature between cells, and reduce pressure loss in the flow path. To ensure the safety of the vehicle cabin if an abnormal situation arises, the pack is designed so that the cooling draft and gas exhaust grooves are entirely separate.
A new custom ASIC (application-specific integrated circuit) with an enhanced self-diagnosis function was developed for use in the battery monitoring system. Each ASIC can monitor four or six cells.
Hitachi currently has production capacity for its third-generation cylindrical batteries of 300,000 cells per month.
| Hitachi automotive Li-ion HEV power cells | ||||||
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| 2nd gen | 3rd gen | 4th gen | ||||
| Status | In production | Entering production in 2011 | Under development | |||
| Format | Cylindrical | Cylindrical | Prismatic | |||
| Cathode material | Manganese (Mn)-based | Improved Mn-based | Newly developed Mn-based | |||
| Anode material | Amorphous carbon | Amorphous carbon | Amorphous carbon | |||
| Capacity (Ah) | 5.5 | 4.4 | 4.8 | |||
| Weight (kg) | 0.30 | 0.26 | 0.24 | |||
| Power density (W/kg) | 2,600 | 3,000 | 4,500 | |||
Fourth-generation and PHEV cells. Hitachi’s fourth-generation HEV power cell offers a power density of 4,500 W/kg. The cell uses new manganese-based materials designed with optimum grain size using microstructure control and other methods. Measures such as making the electrodes thinner were also used to reduce charge transfer resistance.
The fourth-generation cell demonstrates 1.5 times the output power density of the third-generation (4,500 vs. 3,000 W/kg) and good heat radiation performance. When operated continuously at 11 C under natural convection flow conditions, the temperature rise was approximately 23 ºC and the temperature variation within the cell approximately 2 ºC.
Hitachi is also developing a prismatic battery that combines high energy (for EV operation) and high output (for HEV operation) that can be used as a power source for PHEVs. Hitachi was able to meet these sometimes conflicting objectives by using a low-resistance structure for the prismatic battery together with optimization of the electrode thickness and the composition of the active materials. The new cell also features the use of a ceramic separator and development of new electrolyte additives.