<2024 New Edition> Current Status and Future Outlook of Bipolar Battery Technology Development
A single-cell
secondary battery consisted of monopolar electrodes, where both sides of the
current collector are composed of the same electrode material, has all
electrodes immersed in the same electrolyte. Since each electrode is connected
in parallel using external connecting wires, a significant amount of inactive
material has been integrated into the battery system. As a result, it is
estimated that the volumetric energy density may experience a loss of
approximately 40%, and the gravimetric energy density approximately 20%.
The bipolar battery
features a simple cell configuration and shape as it does not utilize
electrical connectors or other accessories. The volume of the battery is close
to the product of the total stack thickness of the individual unit cells and
the substrate area of the unit cell, while the weight of the battery is
comparable to the total mass of all components. Although the capacity of the
bipolar battery is equivalent to that of a single unit cell, the output voltage
of the bipolar battery is determined by the number of unit cells connected in
series and the voltage of each cell multiplied together.
Using bipolar electrodes
in batteries significantly increases both volumetric and gravimetric energy
density. Additionally, based on application-centric design, the battery shape
can be easily adjusted to maximize the utilization of the battery storage space
in the target device. In other words, the battery volume decreases, and by
minimizing the BMS, energy density enhancement and cost savings can be
simultaneously pursued through minimized use of cell packaging materials. This
ultimately translates into the ability to install more batteries in limited
electric vehicle battery mounting spaces, potentially leading to increased
driving range. Therefore, these advantages of bipolar electrodes are highly
attractive for the design of secondary batteries used in mobile electronic
devices and electric vehicles.
Another advantage of bipolar electrodes is that
electron flow occurs vertically through the substrate, and when the substrate's
cross-sectional area is large, current density and distribution are
significantly improved. Therefore, using bipolar electrodes allows
fast-operating secondary batteries to function safely without any safety
issues.
Starting with Furukawa Electric's compact
batteries featuring bipolar electrodes, Toyota has recently commercialized
bipolar Ni-MH batteries, which were applied to the Aqua HEV. In the
announcement at June 2023, Toyota revealed a roadmap stating that they plan to
produce bipolar LFP batteries for volume-grade EVs in 2026-2027 and bipolar
Ni-based LIBs for future versions of EVs in 2027-2028. This roadmap aims to
enhance driving range and reduce costs compared to performance versions of
LIBs.
The recently released Toyota Crown Crossover
and Lexus RX feature an improved version of the traditional Ni-MH battery,
known as the bipolar Ni-MH. This marks a departure from the previous trend of
using LIBs, especially in high-end and fuel-efficient models. This shift
suggests an intention to gradually expand the use of Ni-MH batteries across the
lineup, indicating a strategic change in battery technology adoption.
In this report, we have compiled the history of
the development of bipolar electrodes, which have recently begun to be applied,
as well as the current status of research and development. We have detailed
each development to provide a comprehensive overview, making it easy to
understand the overall situation.
The
strong points of this report are as follows:
① Detailed coverage of recent
technological trends related to bipolar batteries
② Detailed coverage of the development history
and current status of bipolar battery developers
③ Concentrated coverage of the
development status of bipolar batteries at Toyota Motor Corporation
④ Analysis of bipolar battery's
key patent
(The history of bipolar electrode development and key timeline)
(Bipolar Ni-MH batteries equipped in the new model of Toyota's 2nd generation Aqua)