Nilar

Technology

Battery technology advancements occur on two different fronts, the first, and more heavily-focused area, is electrochemical material advancements. The second advancement should be construction type. Electrochemical material advancements concentrate on anode, cathode, separator, electrolyte, and other chemical additives. Improvements in these areas can result in safer, more compact, and powerful batteries. In most cases, these improvements are tested in conventional ways, such as simple test cells manufactured in a standard case size such as an 18650 (cylindrical cell with dimensions of 18mm diameter, 65.0mm length). This way of testing removes variables and normalizes test methodologies to validate changes in electrochemical behavior. In this example, the 18650 cell, billions of units are made annually. Thus, the manufacturing process and cost are well understood - making it an ideal test platform. This being said, there are also many attributes around a cylindrical cell that limit the electrochemical capability from being fully exploited. At some point, for a given chemistry, efforts in these electrochemical advancements start to result in a stable product. Once a battery chemistry reaches a level where it can be considered a stable product, the construction methodologies around that chemistry is the next area where improvements can be made to improve the overall performance.

In general, there are 3 basic construction types: cylindrical, prismatic, and bipolar. The following is an illustration of the differences.

Cylindrical

As Figure 1 shows, an advanced cylindrical design uses rolled up sheets comprised of anode, cathode, and separators. The role is encapsulated inside a “can” and the electrodes are tab-welded to the current collectors. The number (and quality) of welds, current collector width (and thickness), and cell length will influence the impedance and power capability. For example, more welds, thicker current collectors and shorter length will lower impedance. Another advantage of the cylindrical cell is its inherent ability to contain high gas pressures. This becomes important for recombinant batteries that produce high gas pressures during charge or discharge.

The disadvantages of a cylindrical cell include high overhead volume requirements for current collectors and packing two or more cells into an array. In addition to volume losses, the act of rolling the electrode pair put high demands on the separator, thus limiting the types of separators that can be used. This may result in high cell impedance and lower manufacturing yield. Last, as cell diameter increases so does the temperature gradient of the cell from the center to the surface. This can cause stability issues and uneven cell aging.

The volumetric overhead of the current collectors is between 5% and 15% of the total volume for this type of packaging. The cell to cell packing efficiency has a 15% to 25% loss as well, thus the packaging volume to active material is between 20% and 40% of the total volume.

The majority of cylindrical cells have become very popular for portable electronics because of their versatile shape, compact size, and high production levels. This makes them ideal for low capacity applications.

Prismatic

Figure 2 shows the construction of a prismatic cell. In this type of construction, there are multiples of the same electrode connected in parallel. Because prismatic cells take advantage
of paralleling many electrodes, they are the ideal choice for high capacity cells. There are two common methods to introduce the separator. One method is to 'bag' either the
anodes or the cathodes, and the other method is to 'zig-zag' the separator between the electrodes. Bagging the electrode contains electrode materials that may cause soft shorts, however
it is more complicated and time consuming to implement. In most prismatic constructions, to limit the loss of volume that the current collectors occupy, both anode and cathode current collectors are fused together on one end of the cell. This is good from a volumetric point of view, however in high rate applications, this becomes problematic due to uneven current paths. This can cause stability issues and uneven cell aging as well. Last, prismatic cells require more mechanical support for battery chemistry types that generate high internal gas pressures. For this type of chemistry, single cell applications are not common. One of the big advantages of the prismatic cell is the packing utilization. In applications where thermal management is required, an engineered
minimum space between cells can be achieved. Last, there is between 10% and 20% volume overhead that must be allocated to packing the current collectors.

Bipolar

The stacking order shown in Figure 3 defines a bipolar battery. The main advantage this construction type offers is the common/shared current collector. This important feature reduces the volumetric overhead of the current collector and inherently results in uniform current flow across the cell, thus making it ideal for high rate discharge applications. The volumetric overhead is approximately zero as compared to the cylindrical and prismatic technologies. This gives an immediate 10% or greater volumetric density advantage. Uniform current and resistance paths also promotes uniform I2R heat generation. Uniform temperature generation also helps with uniform electrochemical aging translating into longer cell life. The size of the surface area and thickness of the cell are directly related to the capacity of the cell. This limits the capacity to less than 40Ah for practical purposes.

Nilar NiMH Bipolar Design

With over a million hybrid vehicles on the roads today, NiMH has proven to be safe, reliable, and more importantly a stable battery chemistry. Nilar began work to develop a bipolar module to improve the volumetric power density and reduce the overall complexity found in battery pack construction. Many companies seen the potential benefits for developing a bipolar NiMH battery but the obstacles associated with this type of chemistry and construction proved for many to be fruitless endeavors. Understanding the problems and engineering solutions were the primary focus for Nilar to develop a working battery that could compete in the ever growing battery market. In addition to solving the issues of a bipolar NiMH battery, determining the ideal markets to approach based on the battery's performance were identified.

One of the major obstacles for developing a bipolar battery using the NiMH battery chemistry is the presence of potassium hydroxide (KOH). KOH is very difficult to seal in an electrochemical
environment. KOH is commonly used as the electrolyte in consumer alkaline batteries. Early in the consumer alkaline battery business the sign of white plumes (potassium bicarbonate KHCO3) forming around the seal was the tell tell sign of KOH coming into contact with the carbon dioxide in the air. The presence of KHCO3 is a physical sign of leakage, and the loss of electrolyte causes premature failure in the form of separator dry-out. Electrolyte in the separator is the medium for ions to cross back and forth between anode and cathode. If ions cannot transport between electrodes then the battery stops working correctly. Once leaking starts, failure is rapid and abrupt. Determining a sealing technique that would prevent loss of electrolyte required understanding the sealing properties employed by cells that reached an oxidation failure mechanism (the ideal end of life failure mechanism). It became clear that the solution was in material selections, purpose of sealing boundaries, and tolerances.

Validation of the sealing technology lead to higher focus placed on the electrochemical requirements for the battery. Using standard materials utilized in cylindrical cell technology, baseline data was generated on battery testing standards. The results compared to equivalent cylindrical cell data showed improvements to both power and energy densities.

In parallel with testing the electrochemical performance, the module shown in Figure 7 was designed for commercial applications. This module was designed for use as a building block in a pack structure.

When the module is used in a pack configuration, the electrical interconnect doubles as a heat exchanger. This allows for air to travel between the modules, removing heat, without compromising the bipolar current paths. Modules can be stacked to achieve high voltage and the number of cells per module is also flexible.