Hybrid vehicles enter the lithium-ion era (Part 1)

The trend of adopting lithium-ion rechargeable batteries (hereinafter referred to as lithium batteries) in hybrid electric vehicles (HEVs) has begun to expand. Compared with conventional nickel-metal hydride (Ni-MH) batteries, lithium batteries are smaller and lighter, and their output power can be increased, which is the appeal of lithium batteries, increasing the power performance of HEVs. Although there is still the issue of high cost, it has recently dropped to a level comparable to that of Ni-MH batteries, prompting automakers to accelerate their adoption.

Honda released the 9th generation new "Civic" at the New York Auto Show in April 2011. The new Civic has no plans to be launched in Japan yet, but it has attracted much attention because its hybrid model, the Civic Hybrid, is the company's first hybrid vehicle equipped with a lithium battery.

Then in May, Toyota launched the "Prius α", a HEV wagon based on the "Prius" with a larger interior space. The Prius α is available in two models, a 7-seater model and a 5-seater model, and the 7-seater model uses lithium batteries.

Toyota adopted lithium batteries as the engine restart power source in the "Viza" idle stop mechanism model launched in 2003, and also equipped the plug-in hybrid electric vehicle (PHEV) "Prius Plug-in Hybrid" with lithium batteries in December 2009. However, it is the first time that lithium batteries are used for power purposes in mass-produced hybrid vehicles.

Small and lightweight is the charm of lithium batteries

Nissan Motor has launched the "Fuga Hybrid" HEV equipped with lithium batteries in November 2010. Japan's three major automakers have successively launched HEVs equipped with lithium batteries from the end of 2010 to the first half of 2011 (table).

Compared with Ni-MH rechargeable batteries, the appeal of lithium batteries is that they can be made smaller and lighter. The lithium batteries used by the three companies in HEVs all have a voltage of 3.6V per cell, which is three times that of Ni-MH rechargeable batteries with a cell voltage of 1.2V. In addition, compared with Ni-MH rechargeable batteries, the active materials of the positive and negative electrodes of lithium batteries are easier to be coated on the plates with a thinner thickness, thereby reducing the internal resistance. Batteries used in electric vehicles (EVs) can use this feature to increase their weight energy density, and as batteries for HEVs, they can also be used to increase their output density.

The unit weight output density of the lithium batteries equipped by each company in HEVs this time is 3550 to 4000W/kg, which is about three times higher than the original Ni-MH battery of about 1200W/kg (table). In other words, judging from the numerical value alone, the weight of the battery can be reduced to 1/3 when the output power is the same. In addition, although the output density per unit volume is not given in the table, if the output power is the same, the volume of the lithium battery can also be reduced to less than half of that of the Ni-MH rechargeable battery.

However, the three companies have completely different ideas on how to make use of these characteristics in actual products. Nissan uses an "output-oriented" battery to achieve high power performance and fuel efficiency; Toyota needs to equip the battery in a limited space, so it uses a "small and lightweight" design; Honda uses a "balanced" design that can increase output power while achieving a small and lightweight battery. Let's take a look at the actual situation of the batteries of the three companies.

The size of the IPU remains unchanged

The IPU (Intelligent Power Unit) on Honda's new Civic Hybrid integrates batteries, inverters, and DC-DC converters, and is the same size as the IPU used in the original Civic Hybrid (Figure 1). The new Civic Hybrid is the company's first model to use lithium batteries, and is designed with an emphasis on safety in the event of a vehicle collision. It also takes into account that if there is a delay in development time, the IPU that originally used Ni-MH batteries can be immediately replaced. However, as will be touched upon below, the built-in lithium battery is significantly smaller and lighter than the original Ni-MH rechargeable battery.

Figure 1: The new Civic Hybrid's IPU (Intelligent Power Unit)
integrates a rechargeable battery, inverter, and DC-DC converter. The size is the same as the original Civic Hybrid's IPU.

Yuji Fujiki, chief researcher of the 2nd Department of the 3rd Technology Development Office, Automotive Research Center, Honda R&D Institute, who is responsible for the development of the hybrid system of the new Civic hybrid, mentioned the reason for using lithium batteries, citing the characteristics of "balancing fuel efficiency and power performance."

The new Civic Hybrid not only improves fuel efficiency compared to the old model, but also ensures power performance, so there is no need to justify "because it is a hybrid, so...". To this end, Honda increased the engine displacement from the original 1.3L to 1.5L. However, the increase in displacement will lead to lower fuel efficiency. To compensate for this, the output power of the battery and motor was increased, and the motor's assistance and energy recovery during braking were increased.

The new Civic Hybrid achieves 44 mpg (18.7 km/L) in urban, highway and combined mode, compared to the previous Civic Hybrid's 40, 43, 41 mpg (17.0, 18.3, 17.4 km/L), and a 7% improvement in combined mode, "achieving top-level fuel efficiency among sedans" (Fujiki). At the same time, the power performance is "not inferior to that of a 2.0L engine vehicle" (Fujiki).

Battery costs remain the same

The lithium battery used by Honda in the Civic Hybrid is a square unit with an aluminum (Al) alloy as the outer shell material, and a Li (Co-Ni-Mn) O2 (cobalt, nickel, lithium manganese oxide) ternary material as the positive electrode (Figure 2). Hard carbon is used as the negative electrode material. The unit is produced by Blue Energy, a joint venture between Honda and GS Yuasa Power Supply (a wholly owned subsidiary of GS Yuasa).

Figure 2: The square cell used in the new Civic Hybrid.
The collector can be seen at the top.