Nissan LEAF Teardown: Lithium-ion battery pack structure

Nissan LEAF lithium-ion battery

In follow-up to the previous teardown report on new Leaf’s motor and drivetrain, this report presents the details of the battery system. Since the initial launch of the Leaf, the battery has been sourced from Automotive Energy Supply Corporation (AESC) which has collaborated in the development of the Leaf battery through all subsequent upgrades. This report also reviews some of the technological transitions in the development of the Leaf’s battery.

Related reports:
Nissan LEAF Teardown: Powertrain with electromechanical structure, and drive system (Nov. 2018)
JSAE Exposition 2018: Nissan's 40kWh battery pack (Jun. 2018)
Nissan aims to sell 1 million electrified vehicles a year by 2022 (Apr. 2018)

Battery pack specifications

(Dimensions include the flange and the mounting legs)

The battery in the Leaf has been mounted underfloor since the first generation. The size of the battery pack remains unchanged but the total power amount has increased dramatically. In 2010 (first generation) it was 24kWh, increasing to 30kWh in 2015 (minor change), then improved to 40kWh in 2017 (full model change).

Battery pack mounting space. After installation of the pack, the bottom surface becomes flush with the body frame. The dark-colored bands on the surface are cushioning materials placed between the battery pack and the body sheet metal. At the top center is a small window-like opening to serve as an access hole from the vehicle compartment to operate the service plug (*Note). The orange high voltage cable connects to the battery pack.

The battery pack removed from the mounting space. Seen from the front are three mounting legs (the other 3 legs are installed on the opposite side) for fixing the battery pack to the body. The middle area has a hollow trough shape to provide leg room space for the rear seat compartment. The white object mounted in the middle of the hollowed section is the service plug (*Note). The part indicated by the arrow is the breather designed to ventilate air through the internal and external spaces.

The entire battery pack structure has been designed to prevent water intrusion. The breather for ventilation (2 places indicated by the arrow in the picture), the three electrical connectors and the service plug mounted on the top are all exposed to the external environment and securely sealed. (Refer to the section below reference the battery pack casing section.)
Battery arrays are separated, with each array referred to as a module stack.

(*Note)　The circuit of the front high voltage section is connected in the following order: External power supply (+) connector ↔ main relay 1 ↔ rear module stack ↔ service plug ↔ front module stack (left) ↔ front module stack (right) ↔ main relay 2 ↔ external power supply (-) connector. The service plug is used during vehicle service and disassembly and requires removal of the lid from the vehicle interior compartment and requires a 3-step procedure to remove.

The battery pack when the upper cover is removed. Several battery modules are packaged, the front 2/3 is the front module stack (left and right side), the group in the rear horizontal row is called the rear module stack. The cell mounting orientation for the front stack is horizontal (flat), and the rear stack is vertical (perpendicular).	The picture shows each component and their location. Each name for the harness is designated by Nissan.

There are 2 main relays installed on the junction box (left upper photo). The high voltage harness is connected to the Power Delivery Module (PDM), the vehicle communications harness is a terminal for CAN communications between the Vehicle Control Module (VCM) and the PDM.

The battery temperature sensors installed at 3 locations as shown in the photo are NTC thermistors (sensor element which reduces resistance when the temperature increases). There are upper and lower temperature limits for the battery to maintain performance. The temperature data measured by the sensors is sent to the battery controller.

The current sensor is directly mounted on the service plug, which has two built-in elements with different characteristics. The magnetic field generating the current is detected at the core section (black cube in the photo) which generates voltage. The SOC (State Of Charge) of the battery is calculated by measuring the battery current and integrating it in time.
(*Note) The battery SOC is individually measured for all cells.

Battery module specifications

(The L dimension excludes the terminal tip)

Since the first-generation Leaf, the module structure has undergone a series of improvements. The latest module structure is seen as the culmination of all these improvements. In the first design, the 4 cells were packaged in a robust metal case that was practically a sealed structure. Later the side walls were significantly expanded, allowing the packaging of 8 cells, and each improvement resulted in weight reduction and an increase in the energy storage capacity per battery pack.

The battery module with the upper cover removed	The upper cover (seen from the mating surface)	Reference: The first module (Source: Nissan homepage)

The new module houses 8 cells (2 sets of 2 parallel cells x 2 units). The electromotive force is approximately 14.6V per unit, about the same as a lead battery. These cells are stacked using an adhesive. A plastic sheet (black color) is sandwiched between the cells and a metal cover is attached to the module. After affixing with bolts, the module and the electrodes inside will be in a compressed state.

There are no gaps between the cells which are compactly compressed after tightening.	A plastic sheet (black color) is glued to the adhesive surface of the cell of the upper cover by an adhesive compound (white part on the far end).

The battery pack does not have a forced cooling mechanism, but the module assembly method allows for cooling. That is, by pressurized bonding of the laminate type cells with good thermal conductivity, localized heat accumulation in the internal cells is prevented, and generated heat is immediately transferred to the peripheral surface. The low resistance of the internal cells and the large sized, high heat capacity battery enable the gradual rise of temperatures in the pack during high loads, making it possible to achieve natural air cooling.

Battery cell specifications

For a description of the cell structure, refer to the image below. For the cathode material, refer to the table below "Transition of Commercial BEV Battery Transition on Nissan Cars."

The graphite material of the anode uses a laminate structure of multiple layers of carbon atoms. The bonding force of each layer is weak making it possible for Lithium ions to penetrate the layers and function as an electrode.

The glued cell is shown being peeled off. The cell has a compact and simple structure called a laminate type cell, of which the thin film cathodes and anodes and separators are cut, stacked, and sandwiched with laminate film and then sealed.	The same cell as seen from the electrode side. The electrode tab of the cell is still sandwiched between the external electrodes.

In this full model change (FMC), the biggest modifications to the battery cell are the change in the cathode material and the reduction of the internal resistance.

(1) Laminate structure – use of ternary materials
Spinel structure in the old model (*Note), was a major shift from the Mn-Ni type materials. (Refer to the chart below “Transition of Commercial BEV batteries on Nissan Cars”.)
(*Note) Spinel structure: a lattice structure resembling that of a “jungle gym” through which the Lithium ions are passing in and out.

(2) Reduction of internal resistance
According to Nissan’s announcement, the electrode changes and revised electrolyte described above led to a 50% reduction in the internal resistance per unit capacity. This is an important characteristics improvement for maintaining a natural cooling system while increasing the battery capacity (*Note).
(*Note) The battery heat generated is the sum of the heat from the current resistance and heat from chemical reactions. In Lithium ion batteries, the heat of chemical reactions is smaller and the heat generated by current resistance is more dominant, so the reduction of internal resistance becomes effective.

Transition of commercial BEV batteries on Nissan cars

Battery pack

Battery cell

In hard carbon electrodes, the carbon layers are not stacked in an orderly manner and the interlayer gaps are wider, which is ideal for Lithium storage. However, its adoption was discontinued due to difficulties associated with manufacturing and large voltage drops during power discharge.

As shown in the chart above, battery performance improved after three modifications. In each of the modifications, the cathode material was different. The evolution of the cathode material is further explained in detail below:

LiCoO₂ (Lithium cobalt oxide, laminate structure)

Simultaneous adoption on Prairie Joy EV, Renessa EV, and Altra EV
The models were the first to adopt a lithium ion battery. The laminate structure, in layman’ terms, means a structure of ternary compounds consisting of an O layer, a Lithium ion layer, and a Co layer stacked vertically. This crystal structure was not robust and Co was expensive, so the search for other materials began.

(Source: Prairie Joy catalogue, May 1997)

LiNiO₂ (Trials to replace Lithium cobalt oxide with nickel, laminate structure)

Nissan did not adopt the NEC battery.
This was an attempt to reduce cost while keeping the layered structure but by replacing the cobalt with nickel. However, there were many manufacturing issues such as Ni contamination of the Lithium layer and a decrease in battery capacity. Eventually it ended with incremental steps such as partial replacement (Co-Ni mixture) and further Mn replacement with the addition of ternary materials.

LiMn₂O₄ (Lithium manganese oxide, spinel structure)

World’s first commercial production of BEV batteries by NEC Tokin.
The material has a more stable spinel structure compared to the layered structure and does not change dimensions even if iithium penetrates during charging. Furthermore, it has cost advantages over cobalt and nickel. NEC has commercialized the product since 1996, but it was never adopted for automotive applications.

LiMn₂O₄ + LiNiO₂ (Modification with nickel addition to the above material, spinel structure)

Used on first generation Leaf.
The battery adopted on the Leaf is a product made by AESC, a joint venture company between Nissan and NEC. As a result, the technology from NEC Tokin was used to develop the AESC battery. In the Lithium manganese oxide battery described above, due to the efficient electrolysis of the electrolyte, large capacity and long life was an issue. However, with a sufficient dose of nickel additive, it was found that the decrease in battery capacity could be controlled, which led to the full scale adoption of it as the cathode material for mass production BEVs.

LiNi_xMn_yCo_zO₂ (Addition of cobalt = known as ternary material, layered structure)

New usage on the second generation Leaf
In the current Leaf, further increases in battery capacity were required, so a laminate structure that can achieve a high density of Lithium Ion storage has been used. The material is made of ternary material, overcoming the concerns of a laminate structure design by optimization of the cell design to achieve significant increases in energy density, leading to its adoption on the Leaf from 2017 onwards.

The battery pack of the BEV is not a simple container, but is a large and heavy structure. If the battery is not protected from water and collision impact, it can bring serious risks to the vehicle and its occupants. Therefore, the battery pack case is a vehicle component that required new development and design requirements.

Construction as a large structure

The battery pack case is divided into upper and lower structures made of pressed High-Tensile (High Tensile Strength Steel) of which multiple structural components are welded and fastened internally. The whole pack is supported by 3 solid beams and is mounted on the vehicle body by 6 mounting “legs”. After installation of the cells, a gasket is installed on the mating surface of the upper cover and fixed with bolts.

The inner surface of the upper case (cover). A black gasket is installed around the mating surface of the case.	The inner surface of the lower case. Each of the stack boards are fixed on top of the T-shaped cross bars that run to the perimeter.

The modules rest on top of the boards for each stack, and the boards are fastened to the top of the lower case (the picture shows the front module stack).

The images below show many of the internal structural parts and the external mounting “legs”.

Protection of internal cells

In the event of a strong impact such as a crash, although the vehicle is designed so that the body frame absorbs the impact, the battery case itself seems to be reinforced and have an impact absorbing structure.

There is a clearance of 50mm between the side wall of the battery pack case and the module. This is very important as a crumple zone to avoid cell damage in the event of a collision.	At the bottom surface of the battery pack are 3 thick beams that cross in the transverse direction.

Consideration has been given to prevent water from entering the case.

A gasket has been applied throughout the perimeter of the mating surfaces of the upper and lower casing. The gasket cannot be re-used once removed.

There are several studs on the connector plate with bolts passing through the casing. On parts such as the above, a sealing compound (light gray material shown in the picture) is applied on the back surface.		On the mounting portion of the service plug, there is a molded gasketsurrounding the 6 bolts affixed to that upper case, which is designed to prevent water and dust from penetrating the case.

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Keywords
teardown, Nissan, LEAF, EV, battery, lithium-ion battery

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